Bouncers have one simple job, and that is to control the people at the venue, or trying to enter the venue. This includes different tasks, such as IDing people and deciding who gets to go in or who doesn’t. Since they are dealing with drunk people, that naturally also includes using force to stop any altercations or removing a person from the premises, if necessary.
Because being involved in altercations is part of the job, bouncers usually have traits that help them along with this. The big dudes have sheer size to their advantage, and often times, most bouncers have some sort of martial arts training. One huge advantage is that they are sober compared to the average drunken customer.