It's a political thing: if they replaced Caps Lock by Escape by default then emacs users would scream murder. If they replaced it with Control then vim users would probably start looting nearby stores.
And thus, the status quo is maintained.
But seriously, like with almost everything keyboard layout-related it's just there because people are used to it.
One of my favorite things: map caps to escape on tap, and control when used as a modifier. I've got it set up that way on my linux machine and it's amazing. I'm sure you can do it on OS X with Karabiner or something.
I'm still surprised when I see people using caps lock instead of shift for typing a single uppercase character (This being the keyboard peckers who still use one finger for everything).
The fact that you haven't had it enabled for years is probably why you can't understand its usefulness. It's just not part of your workflow because its not available. It does in fact provide a lot of value. As an example, entering uppercase alphanumeric codes is annoying without capslock (switching from holding shift for uppercase letters to remembering to let go of shift to enter a number really slows down entry).
That's not really an example of why something in all caps is useful, though.
The only time I've seen anyone using caps lock is in a call center where people think it's the proper way of entering data into a form (my opinion is it's totally not).
I see this sentiment all over the internet but I don't really understand it. Not trying to be snarky but can you explain why you want the caps lock key gone?
I know personally I don't use it too often, but it's nice to have if I want to type in all caps which happens occasionally. Holding down shift with a pinky for a anything more than a few key presses can be a little uncomfortable so I appreciate the caps lock key.
Can someone fill me in on actual use cases of having this key still?
I have been binding caps lock to escape for many many years.