I used to think China has a lot of redundant ways to hit American shipping but this is getting ridiculous.
I think this particular example shows the PLA training for future concepts and scenarios.
In the future, you may have to deal with a situation with a large number of USVs operating within the First Island Chain and due to their size (smaller radar signature), they could potentially sneak up close enough to the coast to launch surprise missile attacks, lay mines, etc. China has lots of shore defense batteries like YJ-83 launchers, but I doubt they can cover every part of the coast line without any gaps. It may be that a GMLRS unit is closest to the target. You may not want to expend a cruise missile on the unmanned ship if you have a choice to use one or the other.
The PLA will most likely gain capability to extend the of GMLRS strike range out to the First Island Chain from the mainland, in the near future. Maybe the PLA can relegate anti-shipping duties mainly to GMLRS in that area. It would certainly be cost effective. Training to find targets out at sea with UAVs is part of the story, this capability should be useful even now for the PLA MLRS unit's most likely scenarios as backup.
I will be surprised if MLRS has not been mounted on a Type 075 deck for training to hit both moving and stationary targets. Similar in concept with what the Americans are doing with HIMARS on their LHAs. At the end of the day it's about having lots of attack options and the luxury to choose the most optimal one.