@AbuHajaar
The army is expecting to do most of the heavy lifting against other battleships in the case of another state invading. While doing that, they can’t handle smaller, faster ships slipping through, with more important things to focus on. For that purpose, they’re hiring mercenaries to act as a net. Probably.
I assume that heavy battleships can’t move as fast as smaller ones. That said, that’s the end of where my speculation can go. I don’t know why they don’t have enough of their own destroyers or whatnot.
Edit: basing it off of Naval ship speeds, a heavy battleship can only move at about 33 knots, but a corvette can move at up to 60 knots. Interestingly, destroyers can only move at up to 30 knots or so...