Why there is a radiator valve on the floor heating borad?


I have a floor heating, and there is a thermostatic valve, just like the radiator one.

Why is that there?

And why can't we use the tado radiator valve there (if we can't)? I mean, it works perfectly, when the boiler is on, and that valve is closed - there is no flow of hot water and the pump is off (because the pump is connected to power via 35 degrees on/off switch)?

Thank you,