The sun sets and does not shine for part of the day, that’s the primary reason. The secondary reason is because rainy and cloudy days may not provide enough sunlight.
When sun doesn’t shine, the solar panel don’t make electricity so the electricity has to be stored for the night or rainy days or it has to be hooked up to the power grid for those same periods.
Because of night.
The sun sets and does not shine for part of the day, that’s the primary reason. The secondary reason is because rainy and cloudy days may not provide enough sunlight.
When sun doesn’t shine, the solar panel don’t make electricity so the electricity has to be stored for the night or rainy days or it has to be hooked up to the power grid for those same periods.
On cloudy days, the panels make less electricity.
The storage is to provide energy when the solar panel doesn’t get enough light (due to weather or nightfall).
Storage also helps level out the power sent to appliances and inverters, preventing damage due to too much or too little energy from the panels.