Gmail is the email solution of choice for a huge number of Netizens, and that provides a rich playing field for developers hoping to be useful to you by providing tools that simplify email overflow.
Enter Google with its latest endeavor, "Inbox." From Engadget:
If you're anything like us, Google's Gmail has an iron grip on your life. Google's looking to create a whole new iron grip with a new app from its Gmail team, and it's called "Inbox." What is it? That's a good question -- Google's made a demo slash advertisement video that we've dropped below. As far as we can tell, Inbox is a combination of Google Now and your Gmail inbox -- a "smart" inbox, if you will. It combines alike pieces of email (bank invoices, for example), highlights related information (like Google Now alerting you to flight changes, traffic, etc.) and keeps track of your life (it'll give you reminders, among other heads ups). Is this the end of Gmail? We seriously doubt it, but it is Google's latest foray into simplifying email.
It's nice for those times you don't have a full client handy. It's a normal FOSS project, not some clingy 3rd party service. But if you don't need it, cool.