The main one, in my mind, is permanency: as I mentioned in the adjacent comment, stable car-independent communities tend to be built around transportation systems that can’t be easily removed.
(I think there are other benefits, like being slightly more comfortable. But permanency is by far the most important.)
Trams are more capacious than buses because they don’t have onboard fuel tanks, so more space for passengers; and they’re fixed to tracks so they can be significantly longer without worrying about the back swinging out.
Trams are also perfectly level with platforms, so there’s no need to waste time to achieve level boarding for wheelchairs, strollers, luggage and the elderly; buses can spend quite a lot of time kneeling and deploying ramps.
(I think there are other benefits, like being slightly more comfortable. But permanency is by far the most important.)