greatest hits albums are good -not very fan of them though- when they include new stuff not released before: i mean, for example, different versions of the all-time singles, live stuff, recording process... things like that...
if you really like a band and the greatest hits just include all the singles, actually it is "useless" (in my opinion) to have a greatest hits: everybody could make his "own" greatest hits album of his/her favorite band just putting all the singles together and make a CD out of that