Reducing file size on disk is a vital process for backup for example, but one may want to keep the full texture size for later re-injection before final cooking.
This tutorial help you export the texture to png, so you can batch reduce them with an external program and reimport the reduced texture.
If you save the full size texture before resize, you can keep a backup of both texture version and re-import the one you want.
This is specially useful for marketplace addon that often make several gigabits.