Brokers online listings vs actual listings

as a noob starting out, I visit many open houses to both meet potential brokers and to get an idea of what to expect at a certain price range. As a tech geek though, I'm curious, do brokers withhold listings online for whatever reason? Say, in order to favor clients with whom they have a relationship with. Would there be a reason not to have a listing return through this site or the company's own website? I'm certain if you've developed a relationship with a broker, and s/he knows of something new that would interest you, you might get a jump on a viewing -- simply because of the delay from paperwork or bureaucracy involved in getting a listing. But would you get listings that you might never have heard of if you didnt work closely with someone? Thanks