Hi Joan, You sound so humble. Nice.
Why use the transposer?
1-To play in the range (lowest to highest note) that best fits the singer or the instrument playing the solo.
2-Many times the pros change keys in the song to make it more interesting. A good example is Mack the Knief. I think it changes keys four or five times during the song.
3-It's sometimes easyier than relearning the song in a different key.
Have a greta day, John C.