Why do black preachers often have a men standing behind them?

Those are the deacons, usually older men. Think of them like assistants. There are also deaconesses and ushers that serve to pass out flyers and collect money, help with communion and things like that. Their job is to assist with managing and running the service.  if something needs to be done during the service the deacons do it sort of like a bailiff in a courtroom. The deacons are usually men because heavy work might be needed.  And then we also have the mother of the church which is usually the oldest female in the church.  In the black Muslims they might serve as sort of like security as well as deacons but in a Christian Church they serve more like just assistants.  The deacons also have a little bit of status but they really don't have any power.  They're there just as the pastor or preachers right hand men so to speak.  Black churches also have a lot of shouting and things like that so deacons might serve to help people who are fainting because they receive the holy Spirit.

Thayer's Greek Lexicon:

diakonos

1) one who executes the commands of another, especially of a master, a servant, attendant, minister

1a) the servant of a king

1b) a deacon, one who, by virtue of the office assigned to him by the church, cares for the poor and has charge of and distributes the money collected for their use

1c) a waiter, one who serves food and drink

These "preachers" are usually very rich "evangelists".  Those are armed bodyguards  Hired killers. .

Apparently, you've never been to church to see how they operate