What evolutionary advantage, if any, was there in human feet sweating so much (as opposed to, say, the knee)?
The evolutionary advantage is this:
1. Assuming "normal=non-sweaty feet" (which I don't know personally, but just for fun), somewhere along the lineage of humans, a mutation occurred, where you get sweaty feet.
2. You didn't die prematurely
3. You grew up and lived a normal life
4. You're wearing socks and shoes, so it makes no difference between you and the non-sweaty feet humans
5. A mate chose you despite your sweaty feet, and you reproduced more children, some with sweaty feet, some without (maybe)
6. You're fit in an environment that allows sweaty feet humans to strive along with non-sweaty feet humans
7. If perhaps a debilitating virus with no treatment and 100% mortality comes and can ONLY recognize non-sweaty feet humans, sweaty feet humans become the epitome of "survival of the fittest".
8. If perhaps in extremely hot climates, non-sweaty feet will have burning, uncomfortable feet, that could make them unattractive to their mates due to their inability to walk, sweaty feet are necessary for a cooling effect, even though the caveat would be smelly feet, at least they can walk and bear kids.
Congrats, you may now look at the bright side and enjoy your sweaty feet