The difficulty for most people in shopping for their own loans is that they don't know all the right questions to ask and most lenders offer only two or three programs.
A mortgage broker, on the other hand, can submit your loan to many different lenders, have access to many different types of loan programs, can shop around for the best and most competitive mortgage rates and terms available tailored to meet your particular needs. Some charge fees (processing or origination fees); some do not. If you are happy yourself doing the ferreting for the best rate, points and fees and canvassing lenders (made profoundly easier by savvy wired Americans), "have at it".