Originally it was largely about the melt feature and getting that old unstable, warbly sound really easily (per voice, not just an effect over the whole signal. Phonec did it before other synths started adding things like "age" and instability, which now you see in roland, arturia and others. Phonec was like the Boards of Canada synth.
One thing I always appreciated though was that you could also use it as an effect for other things.
