Learning how hardware actually works. I learnt on the BBC Micro, a 32k machine running at 2Mhz, powerful enough to run interesting things, simple enough that you could understand exactly how it worked, the memory map, how IO actually happened, etc. A serial port is great for this; USB or Ethernet, not so much.
Yes, for learning, simple can be better. Years ago as a teen my understanding of the basics of assembly and machine language and how CPUs worked finally "clicked" when I was playing with a very simple software simulation of a 4-bit microprocessor. I think it was called "picoprocessor" or something close to that. While useless as a practical architecture, the concepts I learned in that simplified environment were then easy to apply to real-world systems.
