As often happens in Emacs, I hit the wrong key and stumbled upon a functionality I didn't know about. It turns out that if you press 【k】 while in an Org Agenda buffer you will bring up a capture buffer for the date of the entry at point. How is this useful?
Suppose you have a journal or some other Org file organized as a date-tree and you remember that a couple of days ago you forgot to enter some event. You can go to the Org file and enter it by hand but the date-tree makes getting that correct fussy. It's far easier to bring up the agenda, put the point on the date you want, press【k】and enter the information in the capture buffer as you normally would. The difference is that it will be put in the correct place in the date-tree.
This isn't something I, or you, will need to do often but when you do it's a real win. So much easier than trying to insert the entry in the date-tree by hand.