Its versatility, all within one file format.

e.g. for anyone who hasn't glanced at the Org-Mode table of contents: https://orgmode.org/manual/index.html#SEC_Contents

For managing tasks: org files can be richly structured. So you can organise a task in some nested hierarchy; or with a tag; or with some key-value property. You can also make use of the way you organise notes to quickly search for tasks. Org also provides some useful ways you can view your organised tasks.

Besides task management, it's kindof "2nd best at everything", but all within one file format.

For GTD: Org supports a workflow where you can quickly 'capture' some information; then later organise it.

Org Babel: supports running snippets of code in an org-file, outputting the results. It's more primitive than something like Jupyter, though. (But you can also re-arrange the code snippets into files, supporting a literate programming style). -- I've found this a useful tool for keeping code and notes together.

Org has some support for tables/spreadsheets. I don't use this very much. But, again, useful to be able to embed (and update) within notes.

You wouldn't start using org-mode to get all these benefits. You start using a small part of it. And then you notice some other feature of org you'd benefit from, and figure that part out.

It's just a really solid markup for large plaintext documents with automa(g/t)ic formatting and the ability to embed:

-Arbitrary code which can be executed and which can update/operate on the contents of the document

-calendars

-hyperlinks

-images and other media

-tables which can also act as spreadsheets a la Excel

It also lets you export to just about any format including HTML and pdf via TeX, and it has broad integration across emacs so you can easily invoke portions of org-mode from your other buffers/modes.

In other words it's a really flexible medium for representing and working just about anything in a plain text file. I use it as a bug tracker for personal projects, I use it for self-documenting code ("literate programming", as they say), and I use it for writing prose that I intend to export to PDF such as reports and other things because frankly org is a much saner way to write latex/xetex/whatever.

org-mode works well for me because I already use (and live in) Emacs. Since its just plain text, all the ways I'm used to manipulating text work great in org-mode too.

I also dabble with elisp a bit and that fits in nicely here too. For instance, I wrote a really simple command to "mark this todo as done and move it to the bottom of this list".