Yahoo is insisting its users to have there own sign-in seal for their Yahoo accounts. This approach of having a sign-in seal make sure that you are getting the sign-in page from Yahoo itself and not a forged login page as result of phishing.
Although most of the latest web browsers comes with this anti-phishing elements, the sign-in seal would ensure a higher level of security.
If you are using your personal computer at home or office, you can always have this sign-in seal as symbol of trust.
Also read:Learn to make strong passwords for Bank and Mail accounts from Microsoft