Apart from so called 有型, its main function is invite a vehicle examination (police could ask the owner to bring the vehicle for a check, ordinary citizen could also make a complaint to Transport Department and TD will also do the same thing) that costs you money and time (if you have money, you can use money to buy time)
I tinted my glass because I have a big car with a lot of glass. Also, I place my two kids in the car all the time and I don't want them to get into a super hot car after one day of outing. Not to mention tinting can help protecting UV that aged the dash quickly too.....
When I went tinting, I told them give me the darkest that can pass inspection.... so my tint isn't so dark that I can get in trouble....