Transposing a song (or tune) up tends to "brighten" it, while downward "darkens" it. One might not want to transpose for that very reason, but then it might be the very reason to transpose, too.

About 30 years ago I sang in a choir which tended to do a lot of renaissance music a cappella. Our leader was very prone to change a key by a semitone one way or the other, for the effect. For all but one of us, who were good at reading music by intervals, whatever the starting note, this was no problem. But we had one woman who had perfect pitch and it just about drove her nuts. She was fine if we transposed by a third or so, but a semitone made things really difficult for her.