Justify to the users, give them a reason to use or switch to your product instead of staying with one of the competitors. (edit: a better word would maybe have been maintain)

I don't know how often it really plays a role but I think many google products fit this pattern. Search, Gmail, Docs and many others are not novel ideas - they were just better executed than what we had before.

One might even go on a limb and say that facebook is succeeding because it is a better myspace...