'The Royal Navy 1793-1815' is all about the different battles that took place in the water'...my main concern is why the navy was involved at all? why couldn't they just fight in the land ? Is it because the navies were more efficient than the ground forces or is it to prevent the main lands from destruction?
By the time of the Napoleonic Wars, most of the European nations had overseas empires and trade missions. International commerce was reliant on sea transport (and even what was essentially 'internal' trade often went by sea) because there was no other efficient way of bringing trade goods from the colonies to the home nations. By this time, the European nations had become dependent on this international commerce to support their economies and for funding their military efforts.
Therefore, naval control of the seas during wartime was essential to protect your own country's commerce & transport and to attack your enemy's. By choking off the trade going into enemy ports you could starve them of essential goods and weaken their finances. It could also stop them using the sea to transport their armies and military materiel.
As an island nation, Britain's Royal Navy also served in the added role of being the first line of defence (England's wooden walls) preventing French armies reaching the British coast to invade. The blockade that the Royal Navy put on ports of the French and their allies, served both purposes. It protected British commerce and the British nation by preventing French ships getting to sea.
All of the major sea battles were a consequence of both sides trying to control the seas and, by extension, control trade. The first major naval battle, the Glorious First of June, occurred when a French fleet escorting a grain convoy (from America) was intercepted by a British fleet. Had this convoy been stopped then France would have faced major food shortages which might have brought an early end to the French Republic. The Battle of the Nile was aimed at preventing Napoleon's army from dominating the Levant and possibly threatening India. Both of the Battles of Copenhagen were aimed at preventing the large Danish Navy from bolstering the French (directly or indirectly).
Even Trafalgar was the result of an extended campaign that threatened both British colonies in the West Indies and Britain itself. The Battle of Trafalgar was the result of an order from Napoleon to have the combined French and Spanish fleets support French armies in Italy and Southern Europe. The outcome of that battle gave Britain effective control of the Mediterranean making French commerce there very difficult.
The Influence of Sea Power upon the French Revolution and Empire, 1793–1812, A.T.Mahan (1892)
The Foundations of British Maritime Ascendancy, R.Morriss (2014)