Which is definitely just that, an assumption. Some people write things just because they want to or can, and don't care whether anyone else finds it useful for their own purposes. It might just be something for a portfolio, to inspire others, etc, or again, just because. Lots of things are created "just because".
The decision is left to the end user, use it, or don't.
I'd assume that people who make general-purpose open-source software with the intent of it being used by someone other than themselves.
So read the "have to" as "if you want to have people use and appreciate your work, this is a necessary step".