Why were depth charges important during ww1?

To protect ports from possible enemy submarine attacks.

Explanation:

Short answer, Depth charges were used as weapons in the world war one

Explanation:

elongated answer, Depth charge, also called depth bomb, a type of weapon that is used by surface ships or aircraft to attack submerged submarines. The first depth charges were developed by the British in World War I for use against German submarines.

i'd assume india mainly because of how isolated japan was.   plus it is a whole china away.