"Safety" is also what my guy said. The problem is that in isolation the answer doesn't make any sense: why should I be heating water up to then re-mix cold water? Is that not a waste of energy?

The fuller answer, I think, is that the "raw" hot line additionally goes towards uses that can take scalding water, one or more of the dishwasher and the laundry, depending on whether you have either or both of those in your dwelling. (I hadn't thought of those, since my concern was on the shower. I also can't see the piping, in my place, so it was a black box of sorts.) For uses that might interact with a human, then it's a safety thing, then it makes sense to do the mix with cold. (I believe the other line, the shower bound one, is called the "tempered hot" line.)

My guy also refused to raise it too much, or disable it altogether, citing codes and that it couldn't exceed like 140℉ or so. Which was fine, really, since I was asking for it to exceed 95℉…