It seems like magic. Those musicians up there are singing different notes and it sounds incredibly wonderful. Different notes, yet, they blend as one. It seems like magic. Ever wonder how they do that?
A well-developed sense of relative pitch allows you to hear and recognize relationships between notes (intervals), even if you don’t know the note names.
This is probably why Autotune infuriates so many serious musicians: those variations in pitch aren’t mistakes in the performance; they are the performance. If you clean up those “errors” and quantize everything to the “correct” pitches, all you’re left with is dry, robotic music.
Notes on a page are not music. We musicians sometimes need to be reminded of this. It’s easy to lapse into an autopilot practice regimen, in which you sit down and play your sheet music for a half hour without ever really hearing the sounds you’re making.