By creating a Security Phrase, you are creating an additional security measure to better protect you from scams and fraud. For example, let's pretend your Security Phrase is "Me, you and a Dog named Blue." Now if you get an email which claims to be sent from OldRide but does not contain your Security Phrase, then you can presume that it did not come from OldRide. If however, the email does contain your Security Phrase, "Me, you and a Dog named Blue," then you can be REASONABLY ASSURED it was sent from OldRide.
With a Security Phrase, you can be reasonably assured that the email originated from OldRide. Why reasonable assured? With spoofing and other various types of email fraud, scammers send numerous emails hoping they can reach customers who might fall victims to their scam. However, in their spam email, they would not be able to know or customize their message to include your security phrase. Nevertheless, it is your responsibility to protect your Security Phrase. If you forward an email from OldRide to a scammer which contains your Security Phrase, they are now able to send you spoof emails that appear to be from OldRide. To prevent this from happening, ALWAYS remove your Security Phrase before you reply or forward an email.