Why big-gun ships carried torpedoes

Why were torpedoes fitted to capital ships when hindsight makes them look like a poor idea?

They could finish a damaged enemy, conserve shells, and provide a killing option when the main battery was empty or needed elsewhere.

The torpedo was a useful secondary killing method rather than an inexplicable ornament on a big-gun ship. It could finish a crippled opponent without spending more main-battery ammunition. The weapon also gave the ship an option after shells ran short or while the guns remained occupied with another target. Later experience made underwater tubes look hazardous, but the original tactical logic was the value of an additional decisive weapon.