I see some guys in the gym bring two pairs of gloves to training. If there is a session on the pads, they wear bag gloves which are thin and light. During sparring sessions they switch to wearing a larger glove, eg. 14oz. I understand why the larger heavier glove is required for the sparring activity, but why is the larger glove not used for the bag/pad training as well? What benefits are brought from the bag/pad work by using the thin light glove?
Regularly hammering the heavy bag with heavy gloves will compress and deform their padding, so they won't provide the expected protection (to your opponent), rendering them illegal to use in competition.
Light gloves still give your skin some protection compared to being bare-knuckled - you can train a bit harder and longer. Can also help avoid repeatedly slipping off a sweaty bag. Each to their own though - I prefer bare-knuckle, but do often have to stop before I'm exhausted due to torn or abraded skin.
I believe everyone has their own preferances but for me personally, I like to use the lighter gloves on pad work as I am looking to improve my precision and targeting. During sparring, the heavier (more cushioned) gloves are ideal protection for the opponent (as mentioned by the above contributor).