Many forms of synthesis are now available to the musician, but it wasn't always so. It wasn't until the development of electronic circuitry that musicians had the most powerful creative force at their disposal. Arguably, the first 'electronic synthesizer' was the Theremin developed by Leon Theremin in the 1920s. During the next two decades there appeared various electronic organs and then the RCA Music Synthesizer. Several companies began developing synthesizer circuits although Robert Moog is commonly regarded as the father of synthesis, primarily through the development of voltage control. Moog synthesizers appeared commercially in 1964 and the rest, as they say, is history.
Most early analogue synths, Moogs in particular, were modular in nature. That is, you got a set of synth modules that you had to patch together yourself in order to make a sound. It's a very flexible system, but it means you need to know a bit about synthesis before you can even get the thing to squeak. Many modules ended up being connected in the same way, so a natural development was to produce an instrument with the common routings preset or hardwired. This traded some flexibility for ease of use, but it proved a popular move and most synths, particularly those with a keyboard designed for performance, soon had a degree of hardwiring in them.
Technology has moved on apace since the 1960s. We now have reliable and powerful digital circuitry plus, with modern computers, the ability to emulate synth circuitry in software. Even though there are no voltages, as such, in soft synths, many adopt a pseudo CV system. These include Arturia's Moog Modular V and Propellerhead's Reason, among others. Even modern hardware synths - which are, of course, built with digital circuitry - interface with the user through analoguetype controls. And there are still companies such as Analogue Systems (www.analoguesystems.co.uk) and Doepfer (www.doepfer.de) that produce genuine analogue synth modules
Why? Well, the reasons are three-fold. First of all, the principles of analogue synthesis are easy to understand and fairly intuitive. If you understand the basics, it's fairly easy to get acquainted with any synth - far easier than having to learn a new form of synthesis. However, even alternative forms of synthesis use building blocks from analogue synthesis such as envelope generators and filters. Secondly, there is something very appealing about twiddling dials and moving sliders - these things are fun to work with. Soft synths aren't so hands-on, but they're still fun, usually many times more powerful than hardware instruments and much, much cheaper. Finally, many old analogue synthesizers had a distinctive sound - the Moogs were famous for their 'fat' sound, a result of the filters used in their construction. Devotees claim this character is not found in digital synths, and many software developers have worked hard to emulate vintage synth circuitry in software.