To send emails on your behalf — that's the only reason.
SMTP is the protocol all email providers use for sending. Giving Recurrr your SMTP credentials grants sending access only:
- SMTP credentials do not give access to read your inbox
- SMTP credentials do not give access to your contacts
- SMTP credentials do not give access to your calendar
We recommend using an app password rather than your main account password, because app passwords can be revoked at any time and only grant the specific access they're created for.
See also: Can you read my emails?