I have almost no knowledge in electronics and electricity. I've encountered with the concept of open-drain output recently and I think I got the idea. What I have understood is that it is a transistor connected to an output pin, acting as a sink. Therefore, such pin will always output a zero value. Maybe I simplified it too much and I missed something. Anyway, I was wondering why are this kind of outputs needed. As far as I know, if we were to output a one through such pin, we would have to connect an external pull-up resistor. Why not have that resistor already connected internally?

Thanks for any answer you can give. Also, apart from beign a noob in electronics, I'm new to this forum, so I'm sorry if this question is too theoretical. In case it is, I would apreciate some recommendations of forums where to ask these kind of questions.