Most people are out and about during the day when their solar PV system is producing more than enough power to run the home. This excess power is exported to the grid to be used elsewhere. The home owner comes home in the evening and find they are importing power to meet their needs.
A Energy Storage System (ESS) will store this excess power during the day for use in the evening, overnight and even the following morning (system size depending). So the home owner is not not buying power from the utility company.
ESS can also be used to buy cheap electricity during the night to use when it is more expensive to buy during the day. So when there is little or no sun the owner can still keep their electricity bills down.