Personalized Mailbox

What are the dark green mailboxes for in urban area?

In many urban areas I see dark green mailboxes but there is nowhere to slide mail in. What are they used for? They have the markings of the US Postal Service.

Public Comments

1. storage for the local carriers to pick up a refill for their bags
2. I believe that they are used as lock boxes for letter carriers. It's a place for letter carriers to drop off mail they have collected so that they don't have to carry all of it while they complete their routes.