In the select the missing word exercise both सोते and सोती are given (both should be correct) but I guess only one of them is accepted (which is always the case in that type of exercise).
If the "we" refered to a group of females, it would be सोती, and there's nothing in the original sentence to mark the gender of the subject, so both should be accepted.
they mean "why do we sleep?" as a biological general question?
Because otherwise I dont understand this question...
Or philosophical. Or this question is made by a 5 year old who simply does not want to go to bed.
I've put wherefore do we sleep? and it says is bad whilst both mean the same