The practical difference shows up in what the agent produces before you make an offer.
A general buyer's agent shows you houses, tells you what similar houses sold for, and writes the offer. An investor-friendly agent does that and also builds the income side: estimated market rent for each unit, an operating expense estimate, and a cash flow projection at your financing terms. They'll run a cap rate, which is annual net operating income divided by purchase price, and they'll usually have an opinion about after-repair value, the ARV, which is what the property is worth once the work is done. Some of them maintain a contractor list and a lender list and will tell you which lenders in your market actually do non-owner-occupied loans without drama.
The other half is access. Agents who work with investors hear about properties before they're listed, because wholesalers, property managers and other agents feed them. That's the part you can't replicate from a public listing site.
To your real question, yes, there's a point where the agent is mostly transactional cost, and sophisticated repeat buyers negotiate accordingly. Flat fee arrangements for contract-to-close work exist, roughly 1,000 to 3,000 depending on market and scope, and an agent who does volume with one investor will often take them.
One thing that catches people crossing over from passive: on a direct purchase you're the one holding the inspection deadline, the appraisal, the insurance binder, and the loan conditions, all with hard dates. A sponsor absorbs that for you in a syndication. The agent's real value on a first direct deal is often that calendar rather than the analysis, and it's worth paying for once.