Ordinarily, the pump would run most of the time on your system. But if you had an unexpected problem, i.e. stuck mash, system leak, or other problem, you might want to shut off the pump. In that situation, if you forgot to turn off the heater, you could burn up something or scorch your mash. This interlock would keep that from happening even if you forgot to turn off the heater.
An interlock is just an automatic, fool proof way to prevent something undesirable from happening. They are put into different industrial processes all the time when you want to control how things happen in the process.
If you wire your heater so it can come on only when the pump is running, then congratulations!, you have engineered an electrical interlock.
I have been really happy with my system. The worts run off crystal clear and make fine beer. My RIMS does a good job of controlling mash temperatures and takes a lot of the work out of mashing.
I had to solve some minor issues at first with minor leaks on the suction side of the pump. The leaks were allowing air to be sucked int the system in the form of fine bubbles. Replacing some plastic pipe fittings with brass and stainless fittings solved that problem. I think that the heat caused the plastic fittings to expand and create the leaks.
I have some pictures of it posted on this site someplace.