Why armed merchant ships carried carronades

Was armor important for later age-of-sail merchant vessels, and why did they carry weapons?

Their guns were mainly a deterrent to raiders, not protection sufficient to fight a regular warship.

Merchant vessels still liked to carry some armament even when armour mattered little. Carronades gave them a weapon with which to discourage an attack. They remained in serious trouble against a full warship, but pirates and privateers could prefer an easier target.