[acheron]

For me it's because I've come to rely on the Gmail tabs (primary/social/promotions/updates), which the built-in Mail app just ignored. I could probably duplicate the effort with folders (though I use those also, in parallel to the tabs), but I just started using the Gmail app instead which handled it all fine.

[r00fus]

There's that - and the fact that Gmail app can prevent the non-Primary tabs from sending notifications.

Also there's search... Mail.app does not handle search well for Gmail.