I prefer to type emails out in Drafts due to TextExpander integration in Drafts. I also find that if I type it out in Drafts, I don’t have the anxiety of having to finish it in one go as I do in the Mail app. In Drafts, I can start it, go away, come back and finish it then sit on it for a while. Once I’m ready, I hit the action for email, populate the addresses if I’m not using an action with predefined recipients. I often send emails daily to the same group of people, so I have an action that takes the first line of the Draft as the subject then the rest of the draft as the body of the email. Then puts it in the new email popup from Mail, with the Recipient filled out, Subject, and Body, all I have to do is hit send. Saves a little bit of time, but when you are sending to the same group of people day after day, that saved time adds up.
I hate OmniFocus input on iOS. Worst implementation ever, and as someone who had been using OmniFocus from when it was an OmniOutliner document template, that is saying something. So Drafts eliminates the OmniFocus new task UI, and via actions allows you to type the task or tasks in natural language, then one of the many actions on the actions directory allow you to send the parsed natural language to OmniFocus. No more tapping for individual fields, no more annoying scrolling with the date picker. Actions also allow you to send multiple tasks over at once via natural language. So instead of going through the new task input twice in OF, you write two or more sentences in Natural Language which is much quicker, fire off the action and your multiple tasks are added to OmniFocus.
I don’t use Notes so I can’t speak to advantages there, I use DEVONthink, and it’s nice to use the URL scheme to add text as markdown, plain text, rich text, or formatted document with one action without having to use the share sheet. Also, the ability to append to a specific item in DEVONthink is crucial to some of my daily routines.
Drafts simplifies the process of getting text into other apps. Drafts stand alone wouldn’t be that significant other than syncing text from one device to another, where it stands out is actions. If you are only using the Share Sheet action, then yes there wouldn’t be an advantage to Drafts, but custom actions, even by going to the Drafts Action directory and downloading examples are way more powerful and useful than any share sheet extension.
Take a trip over to the Actions directory and search for OmniFocus and try a couple of actions. I think that Drafts will make a lot more sense to you once you’ve done an immersion trial.