This is kind of a broad question, but I think you should examine what people said about coffee in the early history of the beverage.
"...it drove away fatigue and lethargy, and brought to the body a
certain sprightliness and vigour."
--Abd al-Qadir al-Jaziri, 1587, quoted in The World of Caffeine by Bennett Alan Weinberg, Bonnie K. Bealer.
"A beverage as black as ink, useful against numerous illnesses,
particularly those of the stomach."
—Léonard Rauwolf, Reise in die Morgenländer, 1583, quoted in Wikipedia.
"That which makes for its coldnesse is its stipticknesse. In summer it
is by experience found to conduce to the drying of rheumes, and
flegmatick coughes and distillations, and the opening of obstructions,
and the provocation of urin. It is now known by the name of Kohwah.
When it is dried and thoroughly boyled, it allayes the ebullition of
the blood, is good against the small poxe and measles, the bloudy
pimples; yet causeth vertiginous headheach, and maketh lean much,
occasioneth waking, and the Emrods, and asswageth lust, and sometimes
--Dr. Edward Pocoke, The Nature of the Drink Kauhi, or Coffee, and the Berry of which it is Made, Described by an Arabian Phisitian, 1659. Quoted in All About Coffee by William H. Ukers, 1922.
The following are also quoted in All About Coffee:
"this drinke they take every morning fasting in their chambers, out of
an earthen pot, being verie hote, as we doe here drinke
aquacomposita in the morning: and they say that it strengtheneth
and maketh them warme, breaketh wind, and openeth any stopping."
-- Bernard Ten Broeke Paludanus, 1590s
"...Drinking a certaine liquor, which they do call Coffe, which is
made of seede much like mustard seede, which will soone intoxicate the
braine like our Metheglin."
-- William Parry, 1601
Oh, what the heck, here's a collection of the claims made about coffee around that time, from All About Coffee:
- "Good for the head and stomache."
- "It causeth good concoction, and driveth away drowsinesse."
- "...helpeth, as they say, digestion, and procureth alacrity..."
- "...It is very good to help Digestion, to quicken the Spirits and to cleanse the Blood."
- "...Comforteth the brain and heart, and helpeth digestion."
- "...Wholesome, they say, if hot, for it expels melancholy."
- "...dryeth ill Humours in the stomach, comforteth the Brain, never causeth Drunkenness or any other Surfeit, and is a harmless entertainment of good Fellowship."
So the point here is that you had a lot of people spreading the word about coffee -- suggesting that it drives away fatigue, but also that it had medical benefits. It's a stimulant, and people tend to like stimulants, and this one has a fairly pleasant smell when prepared -- I think word of mouth was probably enough to expand its popularity.