Because every time you don't use the correct fork at dinner, a fussy but delightfully acerbic etiquette author expires. That is why it matters if one uses the correct fork.
We kid. No one -- as far as we know -- has ever been truly assaulted by the use of the wrong fork. In fact, many etiquette authors would point out that calling attention to the error -- much less going into cardiac arrest over it -- would be an extremely unseemly act for any host or guest. So that's our first lesson in fork usage -- nobody worth their weight in know-it-allness would correct you on it. (Which, we should note, puts a wrong-forker in an even more awkward position of just being blissfully ignorant of their wrong forkness for the entirety of their life.)
But in order to answer the question of whether or not your fork matters, perhaps we should start with the necessity of the fork, period. Unlike the knife (which is just another version of human's oldest tool -- the axe) and the spoon (or similar scoop-like implements), the fork hasn't been accepted as an all-around eating utensil since the beginning of cutlery [source: Goldsmith]. Sure, fork (or trident-looking tools) might pop up now and again through history, but mostly they were just used for tiny sweets or to spear large chunks of meat [source: Ward]. (Think more along the lines of lifting the roast rather than eating pieces of it from your own plate.)
It wasn't until around the 14th century that forks started to become a bit more common around Italy. With the help of the royal marriage between Catherine de Medici and French Henry II, they started showing up in French kitchens sometime after [source: Ward]. It was still a slow introduction into the everyday lives of Western peoples. It was only the late 19th century that people began to stop staring at the fork with a sense of vague distrust and declare it useful, if still somewhat fashionable [source: Ward].
So why does it matter which fork you use? The answer, in truth, lies in our very first paragraph. It's just seen as an accepted form of etiquette now. The fork's existence -- whether we're talking the dinner, salad, seafood or dessert variety -- is not really a matter of utility. It is, like a lot of things we use, just a cultural implement that made its way to our familiar table. It doesn't matter which fork you use, unless you're being purely practical. It is far easier to use a tiny fork to, say, delicately pick mussel meat from the shell. But other than that? It's simply a matter of Western manners, which is itself a series of made-up rules that a few cultures follow, with or without understanding them.