Why as a backup power? because basically the inverter is not a stand-alone power plants. How it works is simple power plant that is only able to fill the empty battery.
Try to look at the workings of solar power plants
How it works solar power plants I sort as follows:
Solar panel module>>> Charger Controller>>> Aki Cars>>> Inverters>>> Electric Generating
I try to explain with an easy and simple language you can easily understand based on the sequence:
Solar panel module receives heat from the sun and converted into an empty battery power via the charger controller so that the heat generated by the sun does not damage the battery due to over voltage, then the battery is already charged will be converted into electricity. Tools to change the battery flows into electricity flows is called an inverter.
So that changed the battery power into electricity and solar panels but not the inverter, while the solar panel module serves to charging the battery is empty so that after the battery has been re-filled it can be changed by an inverter to generate electricity
Hopefully this article useful to you