After each match, a good matchmaker gets feedback from both parties about compatibility and suitability. The matchmaker develops a good feel for individual preferences and personalities. Matchmakers frequently do a better job, simply because they are better informed that we can ever hope to be. These are the same people who do well at happy hours, parties, dances, and everywhere else. There is nothing wrong with that, more power to them.
Matchmakers gain an appreciation for the individuality and uniqueness of each person in their group.
They can match people who might never realize they would make a lasting couple. The third difference is responsibility. The dating service recruits as many people as possible. It's your job to find someone in the group you would like to meet who is willing to meet. Even if the dating service already has a hundred people similar to you, and even if those folks are having no luck meeting anyone who is interested in them, that is o.k.