Hi Meech the Fleech... welcome to Oddworld Forums!
Yes, this has been discussed in depth before... but you're new, so you wouldn't have been around when the topic was posted.
I dug back in the archives, and you can find it
here.
In Abe's Exoddus you get invincibility and in Abe's Oddysee you get unlimited grenades.
Although you can't actually kill all Mudokons as you need to save some to progress through some areas, so it's the bonus for saving as many as is possible.
Abe Babe...