The panels still generate power even if they're disconnected as long as they're in the sun.
In a home setup they'll probably just get warm, but if you're making a lot of current you'd want it to not do that.

I think a lot of home setups will switch to a water heater, since that's easy and also a potentially useful way to spend extra power.

I did some googling to try to get an idea of what happens if you just quickly disconnect a solar cell, and things seemed to indicate that it's the inverter that switches the DC to AC that likes it the least.

Regardless of the specific reason, I'm quite confident you need something in the mix to eat the excess power from an underutilized solar plant, because otherwise the electrical engineers who built them probably would have taken the seemingly obvious and easy way. :)