Utilities need fast-reacting ways to deal with rapidly changing generation from sources such as wind and sun. Large users can get a big benefit from reducing their theoretical peak usage, reducing their grid connection charge. Both of these usages are economic today.
Utilities have to have excess capacity available to spin up instantly to meet demand. So you have to compare the cost and maintenance with a natural gas peaker plant. I think there is a place for Tesla's PowerPack to replace the least used peaker plants.
I don't know who Tesla expects to sell to, but I agree that utilities could be very interested. They benefit from smoother demand more than just about anyone.
In many places, the utilities aren't allowed to charge different rates over time (or if they do, it has to be in the form of a discount/incentive vs. the single-rate price, it can't be punitive) even though power costs more to produce at some times. Smoothing out demand by pushing storage out to the edges of the grid seems like it would be a benefit to them.
It might make a little more sense for some consumers due to the excessively high retail rates for electricity in some regions.