It's actually true that denote doesn't do anything that the built in Notes application doesn't do. But by that logic, the built in notes application doesn't do anything that the built in Mail application doesn't do. All denote does is allow you to write a note and then send it to yourself by email. The value is that it does it with the minimum of hassle. You can start writing immediately that the app starts, and then one button sends the message, which is already addressed to you.

For many people, this is useless, but for people who send themselves email as a way of taking notes, this is actually quite a timesaver.

Mail (on iPhone) does not allow to write notes (except as considering unsent mail as note!); but Notes works well in this purpose, and allow to send or not notes, to friend or self. Although it allows to navigate easily through saved notes, and to modify them as far as wanted. I really don't understand what Denote actually does better.

Remember - Denote is designed for people who use their email inbox to store notes. So firstly it saves time by having your email address pre-set. You don't have to select it each time. It also works better than the built-in notes for this purpose precisely because it does not save the notes itself. If it saved them, the user would have to manage them - deleting them from the notes app manually.

Denote is just a simple front-end designed to streamline the process for those who send messages to their own email box. The benefit is its simplicity, not the size of its feature set. If you want an application to manage and edit notes in the style of a notepad, the Notes app, may be better for you - or even the excellent NoteSpark. However some people want a quick way to send mail to themselves without the extra steps needed if they use either the notepad or the mail app.

I hope this explains why such a simple application may be useful to some people.