Early Glocks came with NFML magazines that were designed to swell when loaded. The thumb cutout in the frame made it easier to remove a stuck magazine. All Gen 1 and Gen 2 frames have the cutout. G17 sized Gen3 frames still have it.
When your hands are sweaty, wet, dusty, bloody etc and you're hiding in a ditch doing a combat reload, now is not the time to have to fumble a mag out... The cutouts are designed to help that out a bit. The more grip you have on the mag, the better.