Why old pre-dreadnought hulls made useful auxiliaries

Why were some pre-dreadnoughts converted into repair, depot, stores, ammunition, cargo, or test ships?

Their large stable hulls remained useful, and their triple-expansion engines were relatively economical to run.

A viewer asks why pre-dreadnoughts were repeatedly converted to support roles after becoming obsolete as fighting ships. Their large, stable hulls provided useful volume and a steady platform for workshops, stores, ammunition, experiments, or accommodation. Their triple-expansion engines were also relatively economical to operate compared with machinery intended for higher fleet speeds. Conversion extracted logistical value from expensive hulls that no longer belonged in the battle line.