Every time they do this Taiwan has to scramble their own jets in the air. So most likely it is to test Taiwan's response times and wear out their aircrafts. Each flight of those fighter jets adds to a maintenance cost on top of the fuel costs, which is disproportionately higher for Taiwan considering the size of the economy.
That makes sense. Most of China's military policy seems to be economic. They make cheap subs for defense. US buys really expensive subs so they can attack. They spend some money their military and all the countries around them spend more money on their military. The other countries get poorer since military investment returns nothing so they have to borrow from China for critical infrastructure.
Why would it stop? Not sure how the Chinese military works, but presumably their pilots need a certain # of flight hours to stay current. It's not like there's really a downside for China
