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→Why Use Different Keys?
While many songs have sections devoted to a modulation, it doesn't need to be so obvious. We may want to modulate out of the key for just a measure to play a certain melody we think will sound good over the current chord.
Even if a song does not modulate to another key, it may require being written in a certain key so that it fits the '''range ''' of a certain instrument. For instance, consider a piece written for clarinet, whose range is E3 to G6. The piece features an ascending 3 octave arpeggio run of the tonic chord, ending on the root note 3 octaves above the original note. There are only a few possible notes that this will work for without exceeding the clarinet's range: E, F, F{{s}} and G. If C Major is the key, the clarinet will not be able to play the piece. So the piece is written in a different key, such as E Major.
Even if an instrument's range is not the concern, '''timbre ''' might be. For instance, many songs written for guitar are written in E Minor. Even though the guitar could play the song in any minor key, the composer chose E Minor because the low E is the lowest note of the guitar. He had conceptualized hitting that low E in mind when he wrote the piece, which is deeper and more impactful than hitting the A or E{{f}} above low E.
If a song is written in one key for a certain instrument, but we want to play it with a different instrument, we may consider '''transposing''' the song to a different key. This simply means moving all the pitches/notes uniformly higher or lower. All the relative pitches between notes remain intact. However, now the piece may fit the instrument's range or desired timbre. For guitar, the easiest way to do this is to use a capo. All the chord fingerings will be identical.