What is Synth Voice in the GM Midi list?

I think the main purpose of Synth Voice is to use it in the melody line to give the listener a bit of an idea how the song would sound if sung. It's not meant to replace a real human voice. Normally you would mute the track if singing karaoke to a midi file.
Another possible use (and I've used this trick) is to use Synth voice as a harmony note to sing along with and it gives the illusion of real harmomy backing.

Keep in mind, General MIDI was not invented for any reason having to do with notation programs. It was developed as a standard set of sounds for actual synthesizers - physical devices, usually with keyboards, that make sounds - to include. Lots of synthesizers out there in the real world included sounds designed to sound sort of vaguely like human voices, but were not actually made of recordings of human voices. That's what these General MIDI presets are designed to acknowledge. So any software soundfont that wants to be GM compatible needs to include those sounds. They don't necessarily make sense to use in a notation program unless you are actually writing for a synthesizer as opposed to real human voices, and you want that synthesizer to use that sound.