Here’s why I find Drafts useful/productive: you don’t need to think about how you are going to process the info you capture before you actually jot it down.
In other words, capture the idea, then send it where it needs to go.
To send an email to a regular recipient, I need to:
- launch the Mail app
- tap the “compose message” button
- choose or start typing the recipient’s name
- start typing my message
With Drafts:
- launch Drafts
- start typing
For me, being able to immediately capture my thoughts and worry about addressing and sending later on is the real key value here.
For regular text/email recipients, I have automation set up so I don’t even need to add them, Drafts does this for me.
And set up properly, Drafts can use the first line of the draft as the email Subject, and the remaining lines as the email Body. Again, this makes things super fast to capture a thought and send it on.
Regarding the append/prepend actions, I use these in conjunction with text files on Dropbox. There is at least one default action in Drafts 5 (I believe) that you can use for experimenting with this. You’ll need to pay the subscription to be able to do any sort of customization to the append/prepend action (or add more append/prepend actions, or to make your own).
How I use this is to keep running text file lists on Dropbox, such as an inbox.txt, todo.txt, groceries.txt, ideas.txt and so on.
When a eureka moment occurs, I launch Drafts, capture the idea, then using the relevant action I send it off to be appended or prepended to the associated text file on Dropbox.
Without Drafts, I would need to launch the Dropbox app, find the folder, find the document, tap it, tap to insert the cursor, then start typing. By then my genius idea has evaporated 
And as others have hinted at above, there are “tokens” you can use when building your actions to do things like insert the current date, or time, or both, formatted as you prefer. Great for capturing journal/task entries with a timestamp without having to laboriously type the date/time by hand.
These can be added to your Action workflows so that they just happen when you invoke the action. You only need to set it up once beforehand.
For example, just type out a todo list item, hit the custom “todo” action you’ve built, and it can be set up to append the text from the current draft to a new line on a text file of your choice on Dropbox (or iCloud Drive, etc), with the current date and time automagically prepended to the entry.
In other words, your draft text is:
clean the gutters
and your action workflow adds it to the end of the todo.txt file on Dropbox, with the capture date prepended:
[2019-01-10] organize garage
[2019-01-08] vacuum bedrooms
[2019-01-14] clean the gutters
So essentially, Drafts reduces/removes process “friction” for capturing and processing text.
I think if you just started implementing Drafts in those sorts of situations and workflows, the concept will click for you in short order. From there, you’ll start envisioning plenty of other ways to adapt these and other actions/workflows to your own needs.
There may be ways to append/prepend to Drafts stored within the Drafts app itself, but I do not work with Drafts that way. But it’s flexible enough as of version 5 and the new Mac that you can work that way too I would assume.
But my setup is to use Drafts only to capture the text and send it off elsewhere. Mostly to text files on Dropbox. In particular, a folder of txt files which I use in conjunction with nvALT for editing/organizing on the Mac, but that’s a bit off-topic for the present discussion.
