The same benefit of any other mail client: you don't have to open a tab that connects to the internet to download and sync just enough data to show "that" there

The same benefit of any other mail client: you don't have to open a tab that connects to the internet to download and sync just enough data to show "that" there are updates, but not "what" those updates are until you click through to something that then needs to pull data to show those changes. The real question is "why is this better than a standalone mail client" like Thunderbird, Apple Mail, or even the hot mess that is Outlook. To which the answer is "depending on how you do mail, it might not be better at all. But if you use Vivaldi already, or if you miss the days of Netscape Communicator 4.7, or you wish you had a mail client that got updated based on user feedback more than once every few years, then this might be something worth looking at for you.