If I'd rather sing a song in A, but prefer to play it out of a G formation, a capo on fret 2 is the logical thing to do.
Here Johnny Cash is playing a C form, but because he is capo'd on fret 2, he is actually singing the song in D (apparently).
Same thing here with Neil Young. He obviously wanted to sing this song (apparently) in the key of F, but wanted to use a D formation.
I hope this helps with the OP's question.