I personally do it so I don't bang the gun on walls, door frames, seat belt buckles, etc. It happens a lot when I OWB carry. It's just far more manageable to maneuver IWB. Also, it is far more comfortable to carry it with my shirt tucked in between the gun and my skin than it is to tuck the gun in between my shirt and my skin. I have tried many holsters, and a 100% kydex iwb with my shirt in between the holster and my skin is far more comfortable than it being concealed in an Alien Gear, Cross Bread, etc holster. I sweat a lot less, and it doesn't feel nearly as yucky. If I need to conceal it, I just pull my underwear up a bit and suck it up. Life goes on. Of course, what works for me may not work for others. YMMV.
Oh, I should mention, I would only IWB a slim frame gun like my Glock 43x. The 1911 and Beretta full size can stay outside my belt thank you very much.