Owners deciding whether to engage a broker are usually comparing a fee against a vague sense of value. Naming the jobs makes it a concrete decision. A broker does five things for a seller. Two of them are genuinely hard for an owner to do alone and are what the fee is really buying. The other three are work an owner can do, is sometimes already doing, and can reasonably ask to be reflected in the terms if they are.
The two you are really buying: reach and screening
Finding buyers you would not reach, and separating serious ones from the rest while keeping your identity confidential until you choose to reveal it. Both are hard to replicate privately, both depend on the broker's actual network rather than on their process, and both are why the single best question to ask a broker is who specifically they would approach.
The three you might do yourself
Preparing the material a buyer reads, managing the process and keeping momentum, and acting as the intermediary in the negotiation. All three are real work, all three are within reach of a determined owner, and the third one has a genuine advantage: an intermediary can test a position without either principal losing face, which is harder when the parties speak directly.
What a broker is not
Not your solicitor, not your accountant, and not a valuer, whatever indication of value they give you at the outset. Their interest is in a completed transaction, which is mostly aligned with yours and is not identical, particularly on timing and on price against certainty. That is not a criticism; it is the reason you still need your own advisers.
Questions people ask about what does a business broker do
Do I need a broker if I already have a buyer?
You are then buying process rather than reach, which is a much smaller thing. Some owners still want an intermediary for the negotiation; the fee conversation should reflect what is actually being provided.
Is a broker's valuation a valuation?
It is an indication of what they think it will sell for, given by somebody who benefits from an instruction. A formal valuation is a different exercise by a different professional, and this site provides neither.
How do I judge one before signing?
Ask who they would approach, what has sold in your sector recently, and then work their contract into money at your own expected price. Those three answers tell you more than any credential.