What is this thing on my outdoor wood burner?

Just bought a home (from my brother) and it has a woodmaster 4400. Fired it up but the pumps (2 of them) never turned on. From what I read it looks like most people have the pumps running 24/7 so I re-wired it that way but this module is what I think was preventing them from turning on, that is why I cut it out of the system.

My question is this - what is it and do I need it? It sits atop one of the hot water lines.

I tried to do some digging on woodmasters website but it was down yesterday. I did end up finding the manual to the stove but it says nothing about an aquastat on the installation instructions and in the manual it said the pumps need to be running anytime the burner has a fire going. The only way I could get the pumps going was to eliminate that from the system. The burner has a ETC system that monitors the water temp and controls the draft door and fan and that is all still working properly.

Thanks timbur, I'll keep running the stove with the pumps on 24/7 and won't lose sleep thinking about it then unless someone tells me otherwise. The ETC appears to be doing it's job nicely, turned off the blower once it hit 170.