In many movies (and also drawings) that take place in the 18th century people use wigs. Why? Was it necessary?
Wigs became almost instantly fashionable after Louis XIII started wearing one in 1624 to hide his baldness, and were almost universal for European upper & middle class men by the beginning of the 18th century. Their main purpose was to mask receding or graying hair, and as a fashion item.
At noon dined at home with a heavy heart for the poor man, and after dinner went out to my brother’s, and thence to Westminster, where at Mr. Jervas’s, my old barber, I did try two or three borders and perriwiggs, meaning to wear one; and yet I have no stomach [for it,] but that the pains of keeping my hair clean is so great.
Abroad with my wife by water to Westminster, and there left her at my Lord’s lodgings, and I to Jervas the barber’s, and there was trimmed, and did deliver back a periwigg, which he brought by my desire the other day to show me, having some thoughts, though no great desire or resolution yet to wear one, and so I put it off for a while.
Thence Creed and I to one or two periwigg shops about the Temple, having been very much displeased with one that we saw, a head of greasy and old woman’s haire, at Jervas’s in the morning; and there I think I shall fit myself of one very handsomely made.
Then by coach with my wife to the New Exchange, and there bought and paid for several things, and then back, calling at my periwigg-makers, and there showed my wife the periwigg made for me, and she likes it very well, and so to my brother’s, and to buy a pair of boddice for her, and so home, and to my office late, and then home to my wife, purposing to go on to a new lesson in arithmetique with her.
And also two perriwiggs, one whereof costs me 3l., and the other 40s. — I have worn neither yet, but will begin next week, God willing.
The King staid so long that we could not discourse with the Duke, and so we parted. I heard the Duke say that he was going to wear a perriwigg; and they say the King also will. I never till this day observed that the King is mighty gray.
By and by comes Chapman, the periwigg-maker, and upon my liking it, without more ado I went up, and there he cut off my haire, which went a little to my heart at present to part with it; but, it being over, and my periwigg on, I paid him 3l. for it; and away went he with my owne haire to make up another of, and I by and by, after I had caused all my mayds to look upon it; and they conclude it do become me; though Jane was mightily troubled for my parting of my own haire, and so was Besse, I went abroad to the Coffeehouse, and coming back went to Sir W. Pen and there sat with him and Captain Cocke till late at night, Cocke talking of some of the Roman history very well, he having a good memory. Sir W. Pen observed mightily, and discoursed much upon my cutting off my haire, as he do of every thing that concerns me, but it is over, and so I perceive after a day or two it will be no great matter.
I found that my coming in a perriwigg did not prove so strange to the world as I was afear’d it would, for I thought that all the church would presently have cast their eyes all upon me, but I found no such thing.
Up and found myself very well, and so by coach to White Hall and there met all my fellow officers, and so to the Duke, where, when we came into his closett, he told us that Mr. Pepys was so altered with his new perriwigg that he did not know him.
After dinner came my perriwigg-maker, and brings me a second periwigg, made of my own haire, which comes to 21s. 6d. more than the worth of my own haire, so that they both come to 4l. 1s. 6d., which he sayth will serve me two years, but I fear it.
While the first quote implies hygienic reasons, every other quote is about fashion and personal vanity ("I never till this day observed that the King is mighty gray."). Hans Zinsser, in Rats, Lice and History suggests that wigs, or more accurately the fact that men shaved their heads to wear them, protected them from lice. However, Samuel Pepys doesn't seem to agree, he regularly had to clean his wigs from lice:
Thence to Westminster to my barber’s, to have my Periwigg he lately made me cleansed of its nits, which vexed me cruelly that he should put such a thing into my hands.
Having put him in a way of preparing himself for the voyage, I did go to the Swan, and there sent for Jervas, my old periwig maker, and he did bring me a periwig, but it was full of nits, so as I was troubled to see it (it being his old fault), and did send him to make it clean
Up, and going down found Jervas the barber with a periwigg which I had the other day cheapened at Westminster, but it being full of nits, as heretofore his work used to be, I did now refuse it, having bought elsewhere.
First of all, tt was the age's fashion.
But the main purpose was to cover the unhygienic hair. The general hygiene was really on a low level in Europe from the beginning of the dark ages until the end of the 19th century when people started to realize that most of the diseases can be prevented by simple methods like taking bath, washing hands, and by keeping clean the living space.
You can read related sources on wikipedia of Hygenie and also I would recommend the sources on an another wikipedia link where they describe the 18th century wigs, and in the 16th and 17th centuries section they explain the health issues of the hair in these ages which is valid for 18th century as well.
Adding to the other great answers (upvoted):
I once was told by some guides in an 18th century palace (read: I don't have a good source), that this also was partly a pragmatic thing.
People were allowed to use their own hair to create great hair fashion. There are various reasons, why a wig might have been a much superior choice:
- Their own hair might not have been good enough. Either because of lice or something, or because of not having any hair any more (possibly due to bad medicine).
- Different hair fashion for different occassions: If one wanted to wear a new hairstyle, one could either use ones own hair and restyle now and then, or simply take a different wig out of the cupboard.
- Some festivals/balls started at 3 o'clock in the night. One could either style the hair at the afternoon and not go to bed until it started and possibly take a nap in a chair to not disturb the hairstyling. Well, or take the wig and be done. The balls started so late due to candles: When hundreds of them stopped lighting, one could either stop the ball, try to replace them all at once or schedule the ball so that the morning light comes at the right time.
I have read in a book (I have forgotten the title, I'd need to go to the library and find it again), that it was okay, if ones own hair is even visible next to the wig. This also fits somehow the pragmatic view of things.