There are three main reasons to use a capo:
(1) To transpose a song from one key into a different key.
(2) To play in the original key of a song while using familiar chord shapes.
(3) To simply get a different – often higher – sound for a song. In this example I play a chord progression using the original key of G chord shapes, but after I place the capo on the 6th fret I’ve changed into the key of C#.

Here’s a very simple way to get up and running with transposing. Writing out the musical alphabet keeps everything simple and demystified.