Battery's work ok for regular short term power outages, but going 2-7 days after a storm is not that uncommon every few years in many areas. At which point solar for charging in the day + battery backup is a good solution vs grid + generating capacity.
Basically, installation costs with solar are high, but they don't increase much as you scale the system. Battery's let you scale further, reducing price per watt. You also need an inverted for battery's which you can reuse with solar.
Can you point us to a hospital or commercial datacenter that has solar and batteries and no fuel-based generator?
You would need solar generating capacity well in excess of demand (to serve the daytime demand AND fully charge the batteries on an overcast day to address a multi-day outage). That seems far less practical than a mechanical generator as backup.
Basically, installation costs with solar are high, but they don't increase much as you scale the system. Battery's let you scale further, reducing price per watt. You also need an inverted for battery's which you can reuse with solar.