Emacs

To give you an honest answer: for that feeling of freedom. For most tools there exists a clear distinction between the developer elite and the unwashed user masses, where users only get to have a very curated experience decided upon by the elite. To make it worse, those tools often suffer from what I call "the redesigner syndrome". It's when a new ubermench designer(developer) takes over a part of the functionality, redesigns it to support 1-2 of use cases he's interested in(aware of) and then makes the 10 other long standing ones worse or outright breaks them. To user concerns he smugly answers "well, you shouldn't be even doing that".

Emacs has the smallest distinction between the stuff imposed upon you and the stuff you select yourself. Then, it's so easily customize-able that with practice, when you run into some task that can be automated, you'll have a working solution within minutes. That solution would probably be a Lovecraftian abomination, but you may choose to share it with other hackers anyway, who would then use it as a building brick for even more abhorrent abominations of their own.