The blue check mark was supposed to be a service for those consuming Twitter, so that they can have a bit more security by knowing that there's a higher chance to be following the person or thing they were intending to follow; it was not a service for those having the blue check mark.

That's why I don't understand why they want to charge for it.

Maybe a better thing would be to charge per-1000-followers (or per-10000 or bigger brackets) starting at a given threshold, as long as the account is used commercially, where being a star or influencer also counts as commercial use. But maybe even this is a bad idea, but in my eyes a bit better than charging for the blue check mark.