positive modulation advantage

I don't know exactly, and i don't know if answer to your question, but I write about a function present in a syntetizer with modulazion option and a "switch" called +/-. It's an old Wiltron Synt.

If +, and modulation AM, as modulating level increase, also modulated level increase.
If -, and modulation AM, as modulating level increase, the modulated level decrease.

For positive modulation, when modulation signal goes positive the amplitude of AM signal increases or at FM instantenious frequency increases. At negative modulation the result is oposite.

i suppose that positive modulation is used in AM.
but what abt the application of negative mod?

Television is broadcast with negative polarity. White is low amplitude, black is high amplitude. One advantage, the blacker-than-black sync tips become steady maximum-amplitude pulses, so it's easier to design a receiver AGC circuit. Another advantage, noise spikes appear black rather than annoying white.