What are the motives for covering a song? Coolfer liked the theory of the Village Voice's Nick Catucci: To honor the original, to recontextualize it or because the original isn't good enough.

So why did The White Stripes cover Tegan and Sara's "Walking With A Ghost"? Catucci guesses the cover comes from a desire to "lay claim to the song, as if, played primitively, it proves Tegan and Sara were somehow honoring Jack White." Seeing as how Tegan and Sara is among the most unmentionable rock bands of the decade, Coolfer could be sold on the idea that they chose to cover the song because it isn't very good in the first place.

There are a few other obvious reasons to cover songs. One is for money, as some cities' music scene can't support much else than cover bands. Another is for cred, which would explain all the Velvet Underground and Joy Division covers.

Pretty much unrelated:

• The Covers Project is building a database of cover songs to create cover chains ("a set of songs in which each recording is a cover of a recording by the artist who covered the preceding song").
• Wikipedia has an entry on cover songs as well as a guess on the origin of the term.
• The entry at Reference.com has a few ideas about the origin of the term, plus a lot of history.
• Metallica World has a list of the songs Metallica has covered...and there's a lot of them.