Sure they have to have it to mash in, but why do they keep it going throughout the mash?
It's powered, so no one has to physically do it and it insures good mixing by the end of the mash. The cost for the power is very likely to be less than the cost to fix problems created by incomplete mixing. I don't know
that that's why, but it seems reasonable.
If frequently mixing your mash consistently improves your efficiency and makes you happy then keep doing it. I'm fine either way.