It’s about expanding their ability to project naval power in general, but more specifically, trying to to build effective defensive counters against potential future naval blockades and maritime containment e.g. Island Chain Strategy
That’s oversimplified, and there are other aspects to it, including domestic political cultural ones, but naval power and national security is the most significant.
I am not taking any position on justification, legal standing, or strategic prudence for this strategy.
There are any number of white papers, from both Chinese and American security organizations/think-tanks, that will cover the subject in much greater depth if you’re interested.