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;
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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;
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