ive been playing around with unit sprites a little, but i havent noticed any difference between a unit sprite and an actual sprite. what is even the purpose of putting sprites in a map?

Regular sprites don't interfere with actual gameplay and can cover up uneven ugly tiles while terraining and other stuff. It's just aesthetic.

Unit sprites are really the same thing as a unit in most cases, pure sprites are like the stuff that goes on top of doodads.

Unit sprites can be disabled before the game starts(disable tag) and they are automatically set to their default health, no matter what, so if you set the firebat's health to 1000, the sprite will still have 40/1000 health, if you set it to 1 then it'll have 40/1 health.

Also, if you preplace unit sprites for P12, they will show up in melee.

Also also, I'm pretty sure that Center Location doesn't work on units sprites. This could possibly be useful in some circumstances.

http://www.staredit.net/wiki/Sprites

There's pure sprites and unit sprites.
Pure sprites are just images and can't be modified and don't interefere with gameplay in any way.
Unit sprites are basically the same, but with the important difference that when you set the disable flag in the editor they are more like units than images. You can use them for stacking, or cloaking units using a series of disable and enable trigger actions. However a couple of unit sprites then crash the game.

If you want more detailed information on unit sprites check the wiki, falks homepage and search for related topics.
If after that you still got open questions, we'll be glad to help you out.