Because it is small, cheap, easy to operate, has modern interfaces and most importantly, they are all documented.

This may seem irrelevant, but try manipulating an interface of an old computer without having the documentation that tells you how to send signals to each terminal of an interface.

The great advantage of these small devices is that they have been made precisely for that purpose and that they have not only perfectly documented all their hardware, but there are development kits in several languages to do so

because a Raspberry Pi in many applications that an old computer simply does not have.

For example by the size it occupies and how cheap it is and the energy it consumes is normal that there are projects where you can take buy several Raspberry Pi mount LEDs the software you need and develop it and then use this software within a Raspberry Pi that has cost you 20 € or 30 €.

Mostly you buy it for tinkering and curiosity, as well as to build software and hardware systems.If you have to control for example a door and you have to connect that door to your computer it will cost you more money and more time to set everything up for your old computer than if you simply buy a Raspberry Pi and use the Raspberry Pi to work and to develop a system

then you just copy all that data onto an SD card buy all the parts you need for the system and replicate it as many times as you want

plus tracking a Raspberry Pi is fun while fixing an old computer is not.

Think that the Raspberry Pi also consumes very little battery power