> Why does every website even need to provide its own toggle in the first place?

You can use the automatic value. Some people expect to change the colors of your site without changing their system colors, but you are free to just ignore them.

> How often do people use these, really?

Most normal people don't think about it. Depends on your audience.

But usually more people use them than know how to change their system colors.

> I don't get the "everything needs to support dark theme" thing either tbh

Again, most people don't think about that. Depends on your audience.