$a and $b are place holders for pairs of values from a list that the sort algorithm wants to determine the relative order of. Testing a single value (except in something like Hippo's example) doesn't make sense. In the vast majority of cases we want sort to order a list according to some predicate that determines the ordering of two elements. Hippo's example really just groups elements into two buckets rather than sorting them in a conventional sense.

Bottom line, sort orders a list by some process that compares pairs of elements from the list. There is no redundancy using a pair of variables.