Some table manners were introduced avoid accidents. For example:
* Everyone at table holds their knives in their right hands. This way you don't get entangled with your neighbour's knife, thus avoiding doing serious damage to either of you.
* in Europe you don't cut up all your stuff and put a hand in your lap but keep both hands visible at all times. This way nobody can secretly pull a weapon.
Other customs are meant to make eating more convenient:
* like with the knifes, everyone at table has their forks in the same hand. This way you avoid bumping into each other and pushing the food of your respective forks in the impact.
* elbows off the table allow you to pull your body closer to the table. This way you are closer to your plate and have a better chance to avoid spilling soup or sauce onto yourself.
Again other rules are there to make your eating more pleasant to your fellow eaters:
* no farting, slopping, burping, scratching your privates, killing headlice or other disgusting things that might decrease other people's appetites.
As for the question about salad forks: basically you could use the fork for the main course, but that one is usually a little taller. With the shorter one you can eat your salad more comfortably. The same goes for dessert fork vs. main course fork.
Quite a lot of pieces of cutlery were specially designed for their respective purposes. The fish knife in particular is far superior to a normal knife when it comes to eating fish. The risk of accidentially cutting and swallowing fishbones is minimized by the fishknife's special shape.
Also, at a big dinner, the number of forks and knives gives you a hint as to how many courses there are going to be. This way you are pre-warned and can make sure you don't eat so much salad, fish, poultry or roast that you have no space left for dessert

Btw, there is a limit to cutlery: 2 sets plus 1 salad fork are customary if you put the soup spoon and dessert cutlery on top of the plate. If you prefer to put the soup spoon next to the forks, it's 2 sets plus the spoon.
If there are more courses, new cutlery will be layed out.