[SIG-SOG]

Most 1911's I've seen have a mag that when in the gun is flat / flush with the bottom of the gun. Others I've seen have something on the end of the mag that extends down past the magwell(?).

What is that, and what is it for?

[Jim Watson]

Base pad, bump pad, drop pad... various names.
The purpose is to give something to push against to seat the magazine positively without pinching the hand. Also acts to cushion the magazine when dropped in an emergency reload, but that is not the main benefit.

[Quack]

some mags are that way so they have 8rds instead of 7rds.
the extended base pads also help inserting a mag into a gun that has a magwell.