There are probably different reasons for different users but off the top of my head, a lot of people like the couch as it is a good social board, lots of people have issues in therapy that they think nobody else can understand or people might judge in real life, a lot of people spend a lot of time thinking about T outside session and this is a good place to vent about that, and because people are here more than on other forums,, some people have found like-minded people here, and that's not an easy thing to find in real life.
Also I guess a lot of the other boards are quite specific to a particular disorder or issue, whereas therapy covers a broad spectrum.