Why Trafalgar's frigates stayed on the unengaged side

Why were the frigates on the far side of the battle rather than engaging in the line?

They moved to the unengaged side so they could relay signals between admirals and ships without joining the line-of-battle fight.

Frigates repeated signals between the admirals and the other ships. Their position helped ensure that messages reached ships which otherwise might miss them. They moved to whichever side was unengaged as the battle developed.