Callouses form when your skin is pinched between gravity and the bar. If you learn to grab the bar in such a way that your skin is no longer pinched, you will prevent the formation of callouses. Additionally, using chalk helps improve your grip while also preventing callouses because the bar doesn't move as easily in your hands (as opposed to when they are sweaty). Gloves don't prevent pinched skin, and they cause your hands to sweat--making the likelihood of callouses worse.
Gloves exist for the same reason as pads or manta ray attachments for the bar do on squats. To make things easier and less intimidating to the recreational exerciser. Unfortunately these products introduce their own problems as well.
There are those that argue that since gloves make it harder to grip the bar you should use them for grip strength. I would argue that there are far more effective ways of training your grip, such as using Fat Bar grips (makes the bar 2" in diameter), or doing your deadlifts with an Axel Bar (the whole bar is 2"). Sometimes the simple method of holding the bar at the top of the movement for a few seconds is all you need.
Grabbing the bar correctly:
- Locate the "pad" of your hand. That's the part where your fingers attach to your hand, and where callouses are most likely to form.
- If you are pulling (pull ups, deadlifts, rows, etc.): place the pad of your hand on the bar and wrap your grip around the bar. The skin should be compressed, but when weight is applied, it doesn't pinch the skin.
- If you are pressing: place the bar closer to the palm of the hand. This prevents the bar pinching the pad of your hand in the opposite direction.
In short, just think about how gravity is going to pull the weight whether it's you, a dumbbell, a barbell, or a kettlebell. Grab the bar in a way that when the force of the weight is applied, the pad of your hand does not move much.