It's a bookmark for gmail's website on your default browser. It can be slightly useful if you use a browser without bookmarks.
Exactly the same as your browser Gmail tab. But, you don't need to activate your usual browser (e.g. Chrome) which uses a wide range ressources of your system. Useful if you use other heavy softwares while keeping an eye to your Gmail still lit on. Save CPU memory and preserves the wear and tear of your HDD.
Just a browser bookmark, waste of an application shortcut, and totally useless waste of installation time!
Thunderbird you're fired. Gmail is now my preferred mail provider. No longer do I have to have a desktop and web mail service.