A LOT of singers with excellent delivery and voice may have a limited range available - So IF a song covers a wide range , sometimes a difference of a semitone may render a song uncomfortable ,or even impossible to sing !
IF you want to play Ashokan Farewell on a whistle in the key of D , it is neccessary to play on a G whistle in One Sharp (Concert D is G wgistle A )to avoid octave jumps !