I use the transpose button sometimes myself.

It is usually if my voice is tired from singing a lot or I have a throat infection. Then I know I will not be able to hit the high notes, or use my falsetto. Then for convenience on some tunes I take the tune half a step or a whole step down. That is just so I don't have to think about playing the song in another key, leaves me room to get the crowd going and smile at the girls

DocZ