A lot of electronic mailing is rote. You're sending standard messages to people - sometimes more than one people. What do you do if you want to pluck out a message and send it?
Apple Mail lets you keep messages in the Drafts folder (if you can find it). As soon as you fire off a draft it disappears - to the Sent folder. You can get it back to send it again. Bit of a bother.
Outbox doesn't make you chase your messages around. It saves complete messages as files - with their recipients, priorities, sender address, list of unsent copies, and so forth and so on. And no more chasing. The composition window stays put after you send your message. You can use it again. And again. And again.
Compose your message; fill in the recipients, the subject line; the body; choose your sender address; set the priority, the reply-to, the receipt; now save the message to disk. It'll be there when you need it. Again and again and again.