The U.S. EP-3 recon aircraft were flying off China's coast for a very important reason. China has recently received modern destroyers and new submarines from Russia. The destroyers are particularly dangerous as they carry Sunburn anti-ship missiles. The Sunburn has a range of 300 kilometers and was designed as a "carrier buster." On the open seas a U.S. carrier group can nail such a destroyer before it can launch its missiles, using aircraft or nuclear subs. But the Chinese could have these ships launch their missiles from within a well defended harbor. This would be likely if China were trying to keep U.S. carriers out of a war with Taiwan. Even in port, the destroyers could be destroyed by cruise missiles, but you don't want to do that unless you are pretty sure the Sunburns are about to be launched. To know that, you have to pick up a lot of the radio and other transmissions from these destroyers. Such information can only be obtained from many EP-3 flights over the ships while they are operating. This allows building a library of signals the ship emits when it is performing various functions. It's a detective game where victory goes to the side with the most information. The EP-3s are also listening to and observing the new Kilo class subs China has obtained from Russia. While these subs are regularly tracked by U.S. subs, it's the electronic surveillance by EP-3s that provides the warning that the Kilos are up to something. There are only so many U.S. attack subs in the Pacific and you can't afford to track every Kilo all the time. Naturally, the Chinese don't want us picking up all this information. That is why their fighters harass our EP-3s, and why the Chinese want us to keep the EP-3s out of coastal areas where their new destroyers and subs are training.