Why does a standoff group need a heavy cruiser?

Why is the Glorious Heritage so prominent inside a standoff attack group?

It commands the formation and stands between approaching enemies and the vulnerable standoff attack ships.

The deep standoff attack ships provide the formation's main long-range missile firepower. Those vessels are poorly suited to enemies that survive the barrage and close the range. The heavy cruiser commands the group and fights anything that penetrates its outer defenses. Its prominence comes from protecting the strike system rather than supplying most of the strike itself.