[Al-Khwarizmi]

I think they provide a quite useful service.

As a seller, they can handle all the visits for you - no need to take calls from potential buyers, arrange times with them to show them the house, etc. This becomes almost indispensable if you live far away from the house you're selling.

As a buyer, they also provide convenience - sometimes I've seen 3 or 4 different apartments in two hours, which would almost certainly be impossible if I needed to contact and arrange time slots with 4 different sellers. They also filter properties with unrealistic prices - I have seen many owners who have massive bias, having grown in the house, etc., and will charge like 100K over reasonable prices. A good realtor would probably not even show you that home or warn you beforehand so you don't waste your time.

In my country, they also help with the bureaucracy associated with actually performing the purchase, which is far from trivial - although this can vary per country, I guess.