A few years ago, Valve added "premium matchmaking" to deal with people using multiple accounts (smurfs) and with people who were often banned for using cheats.

To be "premium", you had to associate a valid phone number with your account. At some point, this even became the standard way of playing a ranked game, meaning that you can't play now without a phone number associated with your account.

Funnily though, this has not prevented people from smurfing or using cheats in any way. The cost of a new phone number is ridiculous, so it's this $8 fee for Twitter blue check.