Mailboxes allow you to deposit mail after the postal carrier has passed your house / place of work, or from other areas. Provided you have stamps, they can save waiting in line at the post office, too. Their collection times range from 10 am to after the last post office pickup at 6 pm. Use care that you pick a pickup time that hasn't passed if it is important that your letter be in the mail the same day.
There are two commercial websites that list every mailbox and post office known, as well as their pickup times.
Most mail can be sent from mailboxes without visiting the post offices. The postage can be calculated at the postal service website, and stamps can cover it. For irregular shapes, or rigid objects (e.g. CDs), or lumpy letters, there is a small surcharge one must remember to include stamps to cover. Even priority mail can be sent through a mailbox, properly marked. Packages or letters of any kind weighing more than 13 ounces must not be put in a mailbox, however, if they use stamps. Security dictated that they must be given to a postal employee.