An inbox can be your mailbox, your email inbox, and even a physical tray. But those aren't the only places that stuff lands.
I'm not going into the reasons why you need to empty inboxes, but let me say it's really important. If you don't empty them, you risk letting something slide that you should have dealt with.
The real question is this -- Have you identified all of your inboxes? It sounds simple, but I keep finding more of them. Here are some examples of inboxes I've discovered:
The easy one. It's at the end of your driveway and you check it once a day.
Another easy one. We're all familiar with our email inbox.
You accumulate paper from taking notes, handouts, mail, or even your kids' artwork. Hopefully you have a place to collect these.
Some apps have their own version of an inbox built into them. Evernote has a default notebook that notes are added to. Omnifocus has an inbox for collecting actions to be processed later.
If you participate in any social site you have a feed of information from friends and relatives. You probably don't need to do anything about each post but you'll at least learn what's going on in their lives. The trick here is knowing when this inbox is empty.
The accumulated notification list on your phone is a place that's full of things you need to deal with. The question is this -- Should you be allowing apps to automatically add things to your inbox?
Apps with Badges
Similar to the notification center, you'll have apps that show badges on your phone. Those are just screaming for you to check them and make decisions.
The entire house can be an inbox. If there's something on the floor or out of place, you have a decision to make -- should you pick it up and put it away or leave it? If you leave it, it's the same as leaving emails in your inbox. At some point, you'll need to do something about it.
I'm terrible at this. I'll create piles in random places to deal with later. As an example, I recently realized that I pile stuff on our trash can just outside the garage door. The intent is that I'll take that pile to the woodshop the next time I'm out there. (We won't talk about how often that actually works.)
It's interesting how you'll find inboxes that you've unknowingly been using or that have been sitting. They're just waiting for you to do something.