Joan - In general terms, any musician whether arranger, orchestrator, or performer, will in the main, change key for no other reason than to make the piece more interesting and vital.

It is quite amazing the effect an upward key change can have on any arrangement of a song. It some instances of course, a key change is thrown in as part of an original score. Whilst it's true to say that in the overall music scene, key changes may very well have to be made, to accommodate the key of a singer, the main consequence of most key changes, is to enhance the music.

The transpose button is also used to enable players to enhance their arrangement of a number, if they are unable to actually effect the change of key by playing it. - Colin