It can be useful in cases where you have Time of Use metering. You can charge your battery in the middle of the day when energy is cheap due to solar overproduction and then draw down the battery in the evening when power is expensive. If you have it tied to solar panels you can even charge the battery with solar and then sell it back to the grid in the evening. In a few places you can even do "peak shaving" where the power company will give you a pretty nice bonus for switching your battery into grid-feed mode when the demand is so high that they are considering turning on expensive peaker plants.

Most people unfortunately can't take advantage of this, but most battery packs are also suitable for off-grid use.