I didn't really get it until I started managing emails in mass(or bundles if you want to use googles terms). I get a lot of email that isn't spam that I know at a glance if I need to read it or not. With Inbox I can really quickly archive a huge group of emails that I don't need.

I have also used the pin and reminder feature a few times. I still think it's a weird thing to include with email, but it was helpful.