Could be a variety of reasons why they would bring on fresh water, with costs and efficiency likely to be the main reasons.
The individual warships will have desalination plants, but those cost money to run, so for short duration missions, the PLAN may have opted to not run them and just pump fresh water, via hoses, from replenish ships during UNREP instead. Saves money and also acts as a training opportunity.
It could also be that the replenish ship has newer/bigger and more efficient desalination plants compared to the warships, so the cost of desalination per unit of water is much lower, so rather have all the ships run their own plants, the replenish ship runs its and then supplies the rest of the fleet.
I imagine this will be something the PLAN will do in reverse once they have nuclear carriers, whereby the carrier will supply replenish ships with fresh water, who would then top up the escorts, or the carrier could resupply the escorts directly.
Now, I have only been mentioning costs, but that is not just the PLAN being cheap. Because in practical operational terms, when the fleet is out at sea, the cost savings I have mentioned would be seen most directly in fuel savings since that is what the ships would be burning to run their desalination plants. Thus, cost saves equals fuel savings, and that can have a big, direct impact on the endurance, range and combat effectiveness of a fleet.
Remember, logistics are just as important in winning wars as bullets and bombs, and if you can make your resources last longer, that gives you an advantage in battle.
