On a tactical level, the Battle of Trafalgar is one of the most analysed naval battles in history and there are many books and papers covering the battle itself. Most of the strategic analysis covers the events that lead up to the battle, explaining how and why the fleets came together. As it was the last fleet battle of the Napoleonic Wars, it's easy to assume that the British victory effectively decided the war at sea.
However, an article by Rémi Monaque (a French historian and former naval officer) suggests that there was no strategic purpose to fighting the battle -
Nelson did not save England from French Invasion. In fact, on 23 August, Napoleon, who the day before had been watching out for a sight of the combined fleet on the cliffs of Boulogne, suddenly gave up his master plan, and decided to march the Grand Armeé towards the heart of Austria...The tragedy of Trafalgar that evoked so much heroism and cost so many human lives was thus an unnecessary battle without any strategic stakes.
"Trafalgar:A French Point of View" (A Great and Glorious Victory, ed. R Harding, 2008, ch.5)
The crux of his argument seems to be that because the British and Combined Fleets had come together purely as the result of Napoleon's invasion plan, once the threat of invasion was removed, there was no longer any purpose in the fleets engaging. He dismisses the mission that brought the combined fleet out of Cadiz (supporting French forces in Italy) as "entirely secondary" and so, presumably, of no strategic value to either side.
Although the article doesn't explictly state it, I'm assuming that Monaque supposes that the alternative to battle would have been the continued blockade of the combined fleet in Cádiz (until the Spanish revolt in 1808). From a strategic point of view, would that have had the same outcome or were there stategic benefits to fighting?