The question is- if you are already detecting the submarine through a ship or MPA, why not use the helicopters on board the ship or the weapons on the MPA to kill it? What exactly is the requirement of this system?
In all likelihood this is an indication of the presence of passive sensors such as seabed hydrophone arrays (i.e. SOSUS) which can detect presence of surface or subsurface targets over a wide frontage (SOSUS arrays can span thousands of kms).
Once a threat has been localized (at this stage it may or may not be positively identified, which introduces complexity into the mix), a ballistic missile like SMART can deploy a torpedo in the region at only a fraction of the time it would take a MPA like P-8 to arrive in the location, let alone the much slower shipborne helos.
If SMART is anything like Shaurya, it should be able to attain hypersonic velocity inside the atmosphere (above Mach 7) for most of the trajectory. Time is everything for a submarine interception, and SMART can address a big part of that problem.
Another possible advantage is that it would afford a standoff option to deploy ASW weapons into an area where you lack air dominance (like inside the A2/AD influence of a hostile Surface task force, with a submarine escorting it underwater). It wouldn't be possible to safely penetrate that airspace with P-8s or helos...and as long as the enemy surface ship's A2/AD is up, the enemy subsurface element is free of air threats in that region. SMART can fix that.