The usual use is when the sort key must be derived from the data using a process that takes significant time/resources. The idea of the ST is to precalculate all of the sort keys before the sort operation so you only spend the time once:
@sorted =
map { $_->[ 1 ] }
sort { $a->[ 0 ] <=> $b->[ 0 ] }
map { [ expensivefunc( $_ ), $_ ] } @data;
Doing a naive sort, you would be calling that expensive function twice for every comparison, which would end up being a lot more than when precalculating:
@sorted = sort { expensivefunc( $a ) <=> expensivefunc( $b ) } @data;
