Stumbled over this feature. Why I like it:

  • I can easily see what needs my attention right away.
  • I can choose panel contents rather than let GMail choose for me.
  • Example: in:unreads -category:promotions

3 of possible 5 custom inboxes, "Right of inbox" position option"

To try it go to Gmail Settings > See all settings > Inbox tab > Inbox type > Multiple Inboxes.

Choose "Custom" Inbox Type and configure options