This is especially helpful if you run your own servers. If you are presented with an error message or warning that the signature has changed or does not match the IP/domain you are connecting to you always want to verify manually.
So your e-mail/web client will show you an SHA-1 fingerprint like this:
"Could not verify this certificate because the issuer is unkown" or other reasons such as a mismatch in IP/domain.
It will also show you........
So you've just purchased your SSL cert, renewed it and installed it or maybe you've had it installed and working fine all the time with all other browsers but you've upgraded to a recent version of Firefox and suddenly get the warning "Error code: sec_error_unknown_issuer" error.
This is terrible since if you bought an SSL cert, you are most likely using it for trust purposes for your business and obviously that message will scare away most potential customers.........