1) Immediate benefits to just about anyone. If you have your own solar panels, you need this (or something like it). Even if you don't, you can benefit by drawing more power during cheaper off-peak times.
2) If lots of people get in on this, we'll have the storage capacity we need on the grid to be more dependent on renewable energy. Lack of storage capacity is one of the biggest obstacles to increasing renewable generation.
3) If lots of people buy this, the price per unit goes down, and it's the most expensive component of Tesla's main product, their cars - which are effectively batteries on wheels.