Why armored cruisers entered Japan's battle line

Japan could not afford separate forces large enough for both battle-line and cruising duties. Its battleships therefore needed long endurance, while armored cruisers had to be strong enough to enter the battle line when the fleets met. This was a knowingly dangerous role: an armored cruiser might be sunk or badly damaged while keeping an opposing battleship occupied. The trade bought time for a Japanese battleship to defeat its own opponent and then shift fire to the already-engaged enemy ship. The same logic informed German armored cruisers and later Japanese and German battlecruiser concepts.