Tesla's energy storage enables people to use solar energy at night, without the process being a big hassle. If it gets cheap enough, it will make solar power competitive with fossil power in most scenarios. Currently, the battery installations are a cost-competitive alternative to fossil ways of covering peak load. Calling it
useless is probably a stretch.
Ditto with Tesla's cars; they aren't a drop-in replacement for gasoline cars in all scenarios but I've heard more than one Tesla owner say that they will never buy a gas-powered car again. So obviously that implies greater utility for the person in question than any current gas-powered car.