Why battle squadrons had attached light cruisers

Why did Royal Navy battle squadrons have light cruisers attached to them?

They scouted and relayed visual signals across the battle formation.

Light cruisers gave battle squadrons scouts and additional positions from which to repeat flags. Ships standing back from the battle line could help messages reach vessels whose view was obstructed. Nelson's need for frigates at Trafalgar supplies the comparison. Visual signals still required that support even when radio was also available.