Where are SPF records used?
SPF records are used in large websites that find it important to maintain control of the emails they send out. By using an SPF record, they are able indicate which email servers (IP addresses) are authorized to send emails on behalf of their domain.
The mail servers of receiving email providers can then tell with certainty that the emails were sent on behalf of the website, and then deliver these emails to their customers. This provides certainty about which emails are authorized and which are considered spam.