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.