The 2.5% is buying representation, which is a specific legal relationship, and then whatever service the agent layers on top of it.
Representation means the agent owes the buyer duties, and the exact list varies by state, but generally it covers loyalty, disclosure of what they know about the property, and confidentiality about the buyer's position. If you walk into a listing unrepresented and talk to the listing agent, that agent works for the seller. Telling them you'd go to $260,000 when you offered $245,000 is a thing you can't take back.
On top of that, an investor-focused agent typically brings comps that reflect what a rental or a flip resale actually trades for rather than a general market average, contractor and inspector contacts who show up for a walkthrough on short notice, and a read on which listing agents will actually work with an as-is offer. Some bring off-market inventory, some don't. An agent whose only pitch is pocket listings is a thin agent, because pocket inventory dries up.
On your MLS point: an on-market deal can still be won or lost on how the offer is structured. Inspection period length, earnest money, proof of funds presentation, and how the agent handles the listing agent's questions all move whether you get accepted at $245,000 or lose to someone at $250,000.
The thing to check before you hire anyone is whether they've represented buyers on the specific property type you want. An agent who's closed forty single-family resales and zero four-plexes will miss things on the rent roll and the lease review that a small multifamily specialist catches automatically.
