It's common on services that are attractive targets for spammers or bots.

Creating a new GMail account will require a phone number now, except maybe through a few avenues which are rapidly being closed.

Signing up for popular social media services often requires a phone number.

Signing up for free trials on a lot of platforms requires a phone number.

Everyone knows it's not a perfect measure, but it substantially slows down bot and spammer signups. Even spammers who use these verification services may get an account created, but internally it will be assigned a higher index of suspicion and be more likely to be flagged. When services operate at Facebook or Google scale, they can start to notice when 30 accounts have used the same SMS verification phone number through one of these services in the past N days.