Something like 80% of the traffic in the SCS either goes to, or comes from, a Chinese port. I used to have a better reference but this is the closest I can find in my bookmarks for now:

https://web.archive.org/web/20180726044318/http://www.atimes...

So if economic activity is the metric that supposedly justifies US interest there, then the need to secure its vital maritime lines of communication is even more pronounced for China than for the US.