Plasma Cannons are powerful weapons that are designed primarily designed to be anti AC weapon that can deal large amounts of damage but ultimately is slower firing of the normal Laser Cannon but is typically fairly deadly.
These weapons are largely designed to be anti AC weapons and are the the energy equivalent to the Grenade Launcher and features high drain, attack power and heat production. They are fairly slow firing and have a fairly slow projectile velocity. They are still very deadly and something you don't want to hit multiple times against.
The Plasma Cannon is a deadly weapon although really the Laser Cannon tends to be a better damage as it features more damage and it has slower shot velocity and lower (typically half) ammo capacity, along with it being much heavier. So while it is useful, especially the lighter variants, the Laser Cannon tends to be a better option.