Hi there Lemon and welcome to the forum
For a single infusion mash (the most commom method) I use a range of 62 C to 70C - I never go outside that range and rarely use 62 or 70. Here are two situations when I would use them...
Making a Pseudo-Lager using LloydieP's Krispy Kolsch Recipe
I'll often brew this recipe really low at 62 so as I get a very light-bodied and dry beer - a real quaffer this one

. Mashing this low also avoids the fruity flavours which I find a bit overwhelming in many kolschs.
Making a Very Low Alcohol Beer
If I want to brew a beer even as low as say 2.5% ABV, I will adjust the grain bill to give me the desired low AG and then mash at 70 C. The high temp mash gives me maximum body and lowest attenuation (highest Final Gravity). I will use the same hop bill as the original recipe so as to balance out the often sweeter flavour in a high temp mash.
Most beers that I enjoy I 'll do between 64 C to 66C - lagers, pilsners, pale ales, IPA's and schwartzbier I always brew within this range.
Sorry I can't be of much help. I'm not well-read on all the advanced chemistry stuff (and some of it can be quite contradictory) but have tasted enough beers with others to know that a few degrees off in mashing your beer will be indetectable to most people most of the time. The main thing I do is make sure the thermometer I use is accurate and steal all my recipes from well-written books or brewers I know who have a wonderful knack for coming up with great recipes.
Easy peasy!

PP
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