It was really all just electronics lab equipment tweaked a bit for sound production. Electronic music was a child of recording technology. Once it was possible to convert sounds to electrical signals for storage later, it was only a matter of time before someone would try to make sounds starting with just the electronic signals. Here's a link to Wendy Carlos' version of Bach's Brandenburg Concerto No. 3:
While it seemed shocking to some music aficionados at the time, it was a pretty natural use of the synthesizer. Bach had written a huge body of work for the organ, and the pipe organ is very much like the synthesizer. It's got different ranks of pipes made from different materials in different shapes to mimic orchestral instruments. The names of the pipe styles even come from the German names for the different instruments that they mimic. I recently built a single-voice synthesizer from a kit designed by mutable instruments. Here's a picture of the Shruthi-1:
It's not nearly as imposing as the Moog modular, but it's still a lot of fun. Here's a recording I made of a Walczynski organ prelude that I mad on my Shruthi-1:
