Joan .... This is a good question. ... I've often wondered why someone would use the transposer on an instrumental... I don't use the transposer all the time at all, but will use it primarily if I have a song written in one key and I'm going to sing it in another ... I realize the 'proper' thing to do would be to learn it in my key, but sometimes it's too involved or I'm not going to use it that often ...
I also like the way most ballads (especially the standards) sound in the key of Eb ... and blues tunes in the key of Bb ... for whatever reasons ....
t.