Every kWh your panels make from sunlight that you use immediately (or store "behind the meter"), you don't have to buy from the grid.

And not buying something tends to be cheaper than buying :)

Residential solar doesn't make that much sense from a system point of view - it's a lot more expensive than utility grade solar for the same amount of energy, but with the way the energy market works retail electricity prices are much higher than wholesale prices and that makes the upside of rooftop solar a lot bigger for consumers.

Building it out and maintaining it isn't free. And per a friend who's been selling consumer solar installations for years in the North East and gotten disillusioned: the equipment maintenance, repairability, and replacement story isn't great at the company they last worked at and results in a lot of environmental waste. One of the reasons they left. Of course, this is just second-hand information - I don't personally have much intuition for how widespread the issue is.