Reusing spent mash?

[bruvvamoff]

Hi Everyone

Beginner here, be nice.
I'm making a batch of ethanol using 6kg white sugar, 21L water and one pack of yeast. (total 25L)
Apparently one can reuse the left over slop in the still up to 8 times, which begs my first question.

1) If the yeast and sugar reacted correctly then there shouldn't be any of either in the left over mash.
Why reuse it?

And if there is a good reason for someone distilling the cleanest ethanol possible to reuse it then here is my second question.

2) Is this how one reuses mash?
I distill the mash till it's dead, then tip it back into my fermenting bucket.
I add another pack of yeast, another 6KG sugar, and top up to 25L with water and wait another week.