They are basically there to have a good time. And drinking and gambling are two ways in which guys have fun. There are two main forms of gambling that occur. One form is like a mini tournament. Bowlers get in things called brackets.
Most guys get in three, four, five, or even more.
How these work is, there are eight guys in each bracket. Each game you post your score. You are facing one person each game. If you win you go on, if you lose you are done. You go on to face the winner of one of the other matches.