The reason Buyers and Sellers should use a Realtor is for their expertise. It's difficult to know how hard to negotiate without knowing the local market as well as a Realtor does. A Realtor may be able to save a buyer on the purchase price, or may be able to advise going over the asking price - in order that the buyer is the highest bid and secures the dream house they really want. Either way, the buyer has the best chance for the outcome they want by working with a professional Realtor.
Sellers are well-advised to work with a Realtor because the chances of the Buyer bringing in a Realtor are quite high and do you want the Buyer's interests represented by a professional and your own interests NOT represented? A Realtor can advise you to what's customary in your area, perhaps there are repairs you would be expected to do and others that are "over the top." Having someone in your corner is crucial.
Realtors also know where to go and can advise Buyers and Sellers on a multitude of related services: lenders, home cleaners, yard service, pool service, handymen, contractors, plumbers, electricians, etc.
Multi-million dollar homes typically use top Realtors and they shy away from "discount" brokers as well. Why would they pay top dollar? Think about it, they are savvy with money and they know Realtors are worth the price. That says a lot.
Realtors in Hawaii do quite a bit more than in some other states - they ensure the transaction is properly handled from beginning to end and sometimes are involved well after closing - if needed. The good agents in Hawaii are the best!