A crucially important part of space station - we may even consider this subsystem as defining the space station - is a docking port.
Yes, docking ports could exist on an unmanned system - it may be beneficial to dock with a satellite to boost it to higher orbit, or to do some maintenance. But docking ports were born for manned spaceflight, and they are necessary there. Humans can't be in microgravity for even a few days without having to do exercises. In the flight of Soyuz-9 two cosmonauts spent 18 days in the rather cramped volume of Soyuz spacecraft and had to have serious medical attention after landing. So if you want to stay in space reasonably long time, you have to use large volumes of a space station.
Launching space station separately for each flight usually seems excessive. American MOL station was attempted, and Soviet TKS spaceship was tested - there we could have those space station launched with people - but both projects were closed before manned flights. Space Shuttle was used for prolonged missions, often with Spacelab. We might still see something like a space station launched with people in the form of Starship - it's so big, that it could be considered a spaceship for large crew and also a space station at the same time. At the same time space station grew from one-module Salyuts - and rather unique Skylab, which used Saturn-V to get to orbit - to multi-module Mir and ISS. Not only we got used to stations so large they aren't launched in one piece - we use dockings for assembling those modules in space. We see that for human spaceflight docking ports are very useful.
After evolution in USA and USSR docking subsystems started to standardize - the cooperation in ISS project required that. Now we have International Docking System Standard, and several separate engineering teams in various countries made docking units which can dock to each other. At the same time in ISS another popular standard of docking appeared, Common Berthing Standard, which was used for connecting various modules to each other - and usually not for docking a spaceship temporary. Berthing is different from docking - when you dock two spaceships, usually one is active, and moves towards the other, and another is passive, and doesn't do much, except maybe orienting itself towards the active one. With berthing the incoming spaceship stops in space nearby the station, and a station manipulator is used to grab the spaceship and transfer it towards the docking port.
We still use many types of docking systems. Sometimes we don't need interoperability, like when we know that the only module which is going to ever use this port is this particular one. Sometimes we just need to dock something new and the old standards don't work - Vast company recently unveiled a new, large docking port. Docking ports imply interoperability, and with more and more spaceflight, we'll see more and more of them getting used.