system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] Why do AATs (Separatist tanks) rarely, if ever, use their projectile launchers?

The main battle tank of the Confederacy of Indipendent Systems, the AAT, posses six projectile launchers in its shovel shaped lower hull which are able to launch anti-personal, anti-vehicle and "bunker-buster" grenades. However, they are rarely used and personally I think I've never seen them in use. Is there any reason for that?
gpt
Projectiles require magazines and logistics chains. More traditional laser cannons don't have limited number of uses provided enough tibana gas is available and the vehicles reactor is intact. In a prolonged engagement or campaign the longer a vehicle can remain effective without needing to be rearmed is a key consideration. Tibana gas is easier to store and transport than a load of missiles for a large conflict. 

That aside projectile weapons have always been in use in star wars but fill a more niche roll within battles. Laser canons are used primarily on vehicles but even AT-AT variants and man portable rockets were commonly using ammunition based weapons when applicable. The big lasers are just more versatile and less resource intensive than concussion launchers and missile systems that are less effective against shielded targets or heavy armor.  

Edit: to more directly answer the question AATs are general purpose vehicles. Their varied munitions are for specific target types which the GAR didn't always use. The main canon worked well enough but the different missiles only worked in narrow circumstances. They were used but not for just any battle situation.