A Bouncer's job is to provide safety and security for a business, its employees, and its patrons.
They protect their employer's assets and property against theft and vandalism . They ensure their employer doesn't lose their liquor license by ensuring that no underage drinking occurs. They also do this by not allowing access to intoxicated people, and by cutting off, or bouncing people that are in danger of being "over-served".
Bouncers CAN confiscate fake IDs. They can also take valid IDs that do not belong to the person trying to use them. If someone insists that the ID is indeed valid and belongs to them, they will tell them to call the police and have them verify the ID.
Bouncers monitor the employees and patrons to make sure everyone is getting along. A good bouncer can diffuse a confrontation before it becomes physical. If a fight breaks out, the aggressor(s) will be kicked out first and told to leave the property so that anyone else involved can be kicked out without the fight continuing outside the establishment. Depending on the severity of the situation, they may have to call the police.
If someone refuses to leave the property the bouncer can physically remove them. If the person returns they are trespassing and the police will be called.
If someone assaults a bouncer, they have the right to defend themselves within reason. They cannot pummel someone on the ground for pushing them, but can match their level of aggression until they are subdued. They may even make a citizen's arrest and press charges for the assault.