Quantum computers are still in the "we have discovered the basic science, but the engineering is lacking" phase. If we'd manage to make them work as amazingly well as classical computers we could do some things we're currently struggling with much better. Foremost anything that has to do with quantum mechanical simulations, for example designing new drugs, creating useful protein nanomachines, predicting material properties...

Another cool step up in terms of technology would be the manipulation of black holes. Black holes are, via Hawking radiation, excellent mass->useful energy converters. If we manage to create some small black holes we could for example travel much faster around the universe, without having to create and store dangerous amounts of antimatter.