Why would someone use them instead of going straight to the SBA lender?
There are thousands of lenders. Different lenders have different requirements.
Additionally, some favor/dislike specific industries over others etc.
Based on the brokers experience, they will submit your loan to lender which has the greatest chance of approval for your type of loan.
They will also make sure your application submission has everything needed the first time around, in order to prevent a denial for something insignificant.