Lasalle's here to do one thing and one thing only: save our narrator. And save him he does, pulling the narrator back from the very edge of the pit as he's about to fall in. Sure, he has less than a walk-on role – you only ever get to "see" his arm – but without Lasalle, the narrator wouldn't be telling his story at all, now would he?
It's probably important to mention that General Lasalle was a real, historical dude. Why choose a historical figure? Well, Shmoop thinks it kind of amps up the realism and roots us in the setting: the French invasion of Toledo, Spain and the end(-ish) of the Spanish Inquisition.
(Be careful, though: Poe isn't super strict in the history department: see "Setting" for more on that.)