The reason people use underhand grip is to change the way your rotator cuff is activated. I have torn both of mine, and I have spent a very long time using different grip methods that actually changes the way your shoulder rotates to help with pain and injury.

You can also lose the bar using a regular overhand grip, and most people will even use a overhand grip with their thumbs parallel with the bar or wrapped behind against their index to isolate their chest muscles, because with a full-grip, you are utilizing different secondary muscles to stabilize.

So if you see people using that grip, its not a big deal, they either have injury problems, like me, or are trying to utilize different muscles for an overall build.