Why Tiger could replace Iron Duke as a gunnery training ship

Why would HMS Tiger make a better gunnery training ship than Iron Duke?

Her battlecruiser hull provided more space for modifications.

The question follows a counterfactual in which HMS Resistance joins the Renown class and changes which older capital ships the Royal Navy retains. Tiger might then replace Iron Duke as the gunnery training ship. Her battlecruiser hull offered more space for alterations even if the aging ship was worn. The comparison concerns internal room and adaptability rather than Tiger's value as a frontline combatant.