If you install this Chrome extension, it has the ability to "read and change your data on all google.com sites." Why is this necessary? Seems like quite an overreach.
This permission is required in order to capture text from mail form. Gmail operates under mail.google.com domain. You can examine the extension source code to see that it is not doing anything other than this. It encryptes the content in your browser and sends the encyrpted data to Snapmail servers to get the link.