/1/ Regional intimidation. In this era of unilateral action by superpowers (as the Chinese see it) they want to be able to bully and intimidate smaller regional nations. They can use it as a potential "stick" behind their diplomacy, especially when the USA is seen as over extended and unable and unwilling to come to anyone's rescue. The PLAN is therefore worthwhile even without firing a single shell at bending other nations to the Chinese will.
/2/ The potential to mount a short sharp regional operation, if the USA is otherwise too busy to react with its main force. The PLAN would have to wait for the USN to be busy elsewhere, before, say, supporting an invasion of Taiwan.