Why -5 / +5 semitones
Across pop music, a female solo vocalist's top note averages about five semitones (a perfect fourth) higher than a male vocalist's. To sing the same song, a man typically has to come down about five semitones to reach a comfortable key.
The one-tap preset applies that most-common value immediately. From there you can fine-tune ±1–2 semitones with the slider to land exactly in your own range.