If you have a very straightforward financial situation it's pretty easy to shop around yourself
Yes, but more and more people can't "tick the boxes" of the online/High street providers, so waste loads of time making applications only for them to be rejected.
There are more and more self employed, contractors, freelancers, zero-hour contract workers, those on benefits, those reliant on property income, those with sporadic work histories, etc., who won't get anywhere randomly applying for the popular lenders. They need a broker who knows the market and knows which lenders are sympathetic to each kind of "problem".
I despair with the likes of Halifax, Virgin, etc., when a client of mine (usually limited company freelancer) applies directly for a mortgage. Always the same, they don't understand the model so send inappropriate questionnaires, make the client jump through hoops, and then reject anyway.