Decreases what info Signal needs to collect and retain about a user. When using an SMS verification as a proof, Signal needs to log the phone number, because if they didn't, one spammer could use one phone number to create 10^99 accounts.

When using payment as proof, they can verify that payment occurred, validate the account and then immediately forget about the transaction. One spammer would still have to pay 10^99 times to create that many accounts.

If your concern is "muh phone number", then you can pay and not have to give the phone number to sign up.

It's already the case today (and has been for years) that you don't need to give strangers your phone number to chat on Signal. My username is soatok.45; try to get my phone number if you can.

If you want absolutely no info to be collected, ever, and there to be zero cost on the end user too, be prepared to welcome your new spam overlords. Because the people who will benefit the most from a zero cost signup that only requires a username are spammers.