These are not the same thing. A vocoder or talk box effect modulates an input signal with a carrier signal. It doesn't actually digitally synthesize sung speech

These are not the same thing. A vocoder or talk box effect modulates an input signal with a carrier signal. It doesn't actually digitally synthesize sung speech from scratch. Singing synthesis is different, most notably through Vocaloid. It's like a piano roll for singing. With Vocaloid, you can use preset voices, input MIDI notes and words/lyrics, and actually synthesize the sung speech completely in the box, and with significantly more realism based on how the synthesizer has been specialized to do that exact task. It lets a songwriter who is not a singer to write songs with full vocals and instrumentals that they wouldn't be able to do otherwise, in the same way that a DAW/midi sequencer enables a different way of writing music that doesn't require it to ever be played to exist.