Diki, sorry – But the whole point of key transposing is not just to play in the same key, it’s much more than that.
1-Is the key the music is written in complimenting the range of the Trumpet solo?
2-Is the key in the proper range for the person doing the vocal?

Playing in the same key is just one of the reasons.

DNJ, The Capo can be used on the guitar to change the key you’re playing in, but for the most part they are used to create a chord with open strings. If I play a bar F chord on the first fret all the strings are closed (Fingered). If I place the Capo on the first fret and finger an E chord I have an F chord with open strings. Open strings are used for many types of music, country being one.

Sorry, But I had to say that, John C.