“If you’re sick of typing out the same reply every time someone emails you with a common question, now you can compose your reply once and save the message text with the "Canned responses" button. Later, you can open that same message and send it again and again.
It couldn’t get any easier unless Gmail automatically pushed the Send button.”
In additiona, Canned Responseswas designed so it would work with Gmail’s filters feature. As you know, you can create filters based on various keywords, sender, or recipients combinations (among many others) and now you can use those filters to quickly get the (canned) answer you need and send an automatic reply.