In the beginning the synthesiser as we know it was modular.
There are several reasons for this, some artistic and most technical. Although most people associate Bob Moog with the invention of the Modular Synthesiser, he did have a kindred spirit in California, Don Buchla, who was also designing modular systems in the same time frame as Bob Moog. They both approached synthesis from different angles, Bob Moog was more concerned with making the synthesiser a playable musical instrument, whereas Don Buchla was more interested in creating a system that played itself, accurate pitch control was a secondary concern for Buchla. The analog sequencer was a Buchla invention as Bob Moog later admitted ( the Moog 960 sequencer was their version of an earlier Buchla one ).
There were other philosophical differences between the two approaches to synthesis, on a Moog Modular all the connection sockets are quarter inch jacks, whereas a Buchla Modular has a combination of jacks and a connector called a Banana Plug.
On a Buchla, the jacks were meant for audio signals, both in and out, and the Banana Plugs were for control voltages. Don Buchla believed this made the patching of sounds easier as there were two easily identifiable paths, sound and control.
Bob Moog’s appproach was that anything could be plugged into anything else, all the connections were on quarter inch jacks, and the signal levels designed into the system meant that no damage would result from weird patching arrangements.