I know it may sound strange if you are used to modern consumer electronics, but in order to do anything with those "dumb" serial cables you definitely needed protocols. They never "just worked", it was up to the user to properly set up them up and the level of difficulty depended on the type of device. A lot of times this was non-trivial.
For mouse, the most common choices were Microsoft vs Logitech, and later there were the "scroll wheel" variants. Less common drivers also existed, especially for touch screen devices. Windows had the device discovery wizard which would try to discover the mouse by sending data specific for each mouse type and seeing if right response came back, but it only worked for the common protocols.
For networking, the most common choices were SLIP and PPP. If you were doing dial-up, you also had a modem driver to worry about, as well as a way to switch to "internet mode" (nether SLIP nor PPP had a way to ask for username/password for example.. so there was generally a "script" of some sort to authenticate)
For serial terminals, the basic text was easy, as long as you get your serial port speed and flow control right. But the moment you want something slightly advanced, like clearing the screen or having TUI (text-based user interface), you had to make sure you got terminal type (protocol) correct. Nowadays, it's most likely would be VT102, but in the past I have seen VT52 and Kermit. There were also BS key setting and codepage which needed to be set up separately in order for things to work.
For all other devices, there were basically no hope, unless you had a manufacturer-provided driver. For example, I own a serial printer, and it does not support sending any complex data back to the computer at all, all there is is a single "error" line. The PC has no chance to auto-detect the device at all. (Funny effect of this: if I were to connect it to the old Windows PC and run device discovery, it'd print some junk as Windows tries to probe it as mouse, modem etc.. )
Now some time after, PS/2 came out, and it had standard protocols for mice and keyboards -- which made it widely popular, as there was no need for manual configuration.
And finally when USB appeared, it supported many more "device classes", including things like network cards, video cameras, and joysticks; and other devices at least had a standard protocol to announce their type (VID:PID pair), so that OS could automatically pull up the right network driver.