Earlier quoted context omitted.
(I suspect it syncs with the grids 60hz...) Yeah, that's the actual reason. I've seen someone use a small Honda generator to generate the frequency required to trick the solar generator into going... it's a weird setup because you basically tie in your generator using a breaker, so both generator and solar can be enabled at the same time.
That sounds borderline stupid, is there a technical reason you can't just generate the "clock" internally? Do most of the devices require grid AC?
It simplifies the system a lot, because the grid provides "instantaneous" load matching for your house. If the inverter is sourcing more current than your house is sinking, the grid will sink it. If the house is sinking more current than your inverter is sourcing, the grid will source it.
When there's a utility outage, you need something else to provide the load matching. Either a local generator, or a battery is common; but either way, it's a lot more expensive.