For a new buyer, the broker can help with the details of the sale, provides a buffer between you and the seller, and may have knowledge of something you are interested in coming on the market.
A common scenario, a boat owner wants to buy a new boat that is larger or more expensive than his current boat. But he has to sell his boat first or at about the same time. If the boat is one you would like, having you as a buyer will help the broker make the sale on the more expensive boat. Everybody is happy.
Don't expect the broker to go to great lengths to find you a boat or to spend a lot of time explaining the differences between boats. Remember he works for the seller not you. If the broker takes a liking to you he may send out an email to fellow brokers in the area letting them know he has a buyer for a particular model of a boat.
Do your homework. Know what you want to do with your boat, daysailing, short cruises, weekender, travel the world, etc. Narrow down your list of possible boats. Read reviews. Practical Sailor, Good Old Boat, and Sailing magazines all publish reasonably good reviews of older boats. Find and join owners' groups and listen to the stories and concerns. If you're interested in a Catalina, follow the Catalina Forums here on SBO. As a new old boat owner, consider a subscription to Good Old Boat, in addition to the monthly reviews there are lots of articles on restoring and maintaining old boats.
When you are looking for reviews, learn to sort the wheat from the chafe. There are folks out there who present themselves as experts, who aren't really all that knowledgeable. Get your information from multiple sources, the reliable ones will soon stand out.
Find a mentor, in person is best or online. Someone who will guide you through the process and the multitude of questions you will have.
And enjoy the process as frustrating as it may be at times.