Some are to learn, well I'll say to make your life a little bit more educative and fun
They mainly just help pass the time. However, if you're with friends, and a little bit of booze, they can be really fun.
video games can increase your reaction time, hand eye co-ordination, and puzzle solving skills. Depending on the games you play. I.e. Titanfall is good for reaction time, pretty much every game is good for hand-eye co-ordination, and zelda is good for puzzle solving skills.