Why transpose?
To fit a song to your voice: if the melody goes too high, move it down a semitone or two. To play along with another instrument, or to get easier chords with a capo.
Using a capo
A capo on the 3rd fret raises everything by three semitones: you keep easy chord shapes and the guitar sounds three semitones higher. The transposer works out the position that leaves you the most open chords.