Question about grenade launchers vs. autocannons.

I'm writing a screenplay that has to do with Russian military helicopters and I was reading that the MIL MI-8, can either have a gun pod that carries the AGS-17 30mm grenade launcher or a gun pod that carries the twin barrel 23mm autocannon. Now since the twin barrel autocannon probably has a much longer range and more destructive fire power, cause it's two barrels, why would they bother to have a pod for the grenade launcher? Especially since they are of a similar calibre? What's the advantage? Thanks.