Things you can do with diodes

While diodes alone are not suitable for complex logic, they were instrumental on making computers cheaper in the late vacuum tube era. Vacuum tubes have fairly low reliability and short usable life so having too many of them in your computer is really bad for the cost and reliability of your system. Early transistors were not much better. They would get better over time, but cheap, reliable mass produced diodes were available long before transistors got there.

And while diodes alone cannot do it, a system with a few vacuum tubes to provide the gain and driving a whole lot of diodes made a lot of computers possible at price points that vacuum tubes alone could only dream of. An example is the hacker folklore sweetheart LGP-30, of The Story of Mel fame. 113 vacuum tubes driving 1500 diodes made for a computer that was the size of a fridge, weighed 800 pounds, drew 1.5kW and cost $50k (~500k in modern money), which made it pretty much a personal computer for the late 50's.