What's the point of the OUT message on an Exchange?

I don't understand what the point of using the out message is. It doesn't
copy headers. It doesn't signify the end of a route as far as I can tell.
It's not used for a reply message. I don't think any of the official Camel
components even use the out message. What good is it for?

In my own project, I've created a final processor that copies the in
message to the out message and selectively fills in headers that were
configured for that processor. I don't even know why I do this other than
as a way to get rid of the myriad JMS headers that get passed around from
using JMS (and occasionally rogue HTTP headers returned from a SOAP call
somewhere).

Are there any good resources or explanations as to the point of the out
message? It seems to complicate things as it doesn't seem as though you can
send a dual-message of anything by using both the in and out messages.