Brokers should work on the client's behalf, not that of the insurer.
Let me give a very recent example of how we helped a client who crashed his classic Alfa Romeo into the armco barrier while entering a motorway slip road.
He reported the claim and the vehicle was recovered to his preferred bodyshop. The insurer's assessor went out to look at the damage and judged it to require about 30 hours work to repair. They anticipated that it would be a straight bolt off, bolt on repair of a new nose cone and front wing plus a bit of spraying and blending. They authorised the repair at a fixed ceiling amount.
However, the bodyshop, and in this case the client, both know that the assessor was way off. It was more like 80 hours' work and the parts could not simply be bolted on like with new cars. Welding was needed, sealing and rust-proofing on top of the work the assessor accounted for. The client submitted the estimate and the insurer refused it. We took it up for the client and negotiated with the insurer to get them to agree the revised schedule of works and costs.
You try getting an insurer to more than double it's projected repair cost on your own and see how you get on
We have other examples such as a windscreen claim where the repairer couldn't locate a replacement screen because it was a rare model vehicle. Our efforts resulted in a new screen being shipped from the manufacturer in Italy.
Other more mundane cases have been where the insurer just seems to be dragging their heels or they're offering a low amount in settlement and the client doesn't seem to be getting anywhere. We've stepped in and got things moving.