Most of a rocket's weight is fuel. Ironically, most of that fuel is needed to lift the rest of the fuel from ground level to some altitude A, then the remaining fuel is used to get to A', and so on.

If you could first lift the rocket to a high altitude, you could get away with far less fuel. The smaller mass would mean faster acceleration (think a rocket fired from a jet fighter).

note: I'm not a rocket scientist or physicist :-)