Thanks Jethro_ :)
Your right it's not only useful , it's needed. Almost no major websites can be authenticated to without hmac.
For those don't understand what this is used for, it will provides oneway encoding of a key and your data.
The data will be sent to the server but the key never will. You will just send the data along with the signature made from the hmacsha1 of (key,data) .
Since the server already has the key, it can confirm that the signature is correct and accept the data.
Since the signature is sent instead of the key/password, there is never a chance that your key/password could get snatched by net sniffing.

Since this is so much more secure than sending the password with mime encoding , many websites and most social networks have switched to this method.