The logic is that the signature must be replaced when you change from addresses. In order to do this, I have to wrap it in a pattern that allows the add-on to later find your signature in the message body whether it's at the top or the bottom and replace it seamlessly.

Imagine that you write a long email, only to realize that you forgot to change to the righ from address. This way, when you do change, it gets replaced "in place" without being re-inserted above all of the text you have just entered. Another scenareo where this is useful is in editing drafts. This way any changes you have made to your signature will automatically be applied "in place" to your old draft message, even if the draft was created before you made the signature changes.

The "re-insert sig" button simply allows you to re-insert the signature in case things get messed up. Many people don't even want to see this button, and I wanted to clean up the interface, so it must be enabled in the options.

The signature is currently wrapped as follows:

<blockquote>your signature</blockquote>

This is the only non-visible text wrapper that Gmail doesn't strip. I have tried using <div id="bc_sig">your signature</div> but Gmail strips the id="" part, and if I just say "replace everything between '<div>' and '</div>' more than just your signature would be replaced. One quirk that happens here is that if you are emailing back and forth with someone else that is also using the add-on, their signature will be replaced with yours and vice-versa. I am working on finding a more unique non-visible wrapper pattern that doesn't get stripped when Gmail saves a draft, but it's slow going.