What's the deal with the bonus tracks?

I am a big fan of Elvis Costello (pictured [somewhat smarmily] left), and if there is anything that has defined his albums in the CD era is the inclusion of BONUS TRACKS.

For example, as I'm typing this, Elvis's first album, My Aim Is True (pictured right), has been issued four times on compact disc, and by the time My 3 Addictions is released, there will be a fifth version. What is it that makes hapless fans buy an album up to five times on CD (and who knows on how many other countless previous formats)? Besides losing them to your former spouse in the divorce, the number one answer is BONUS TRACKS.

Now, you might be saying, "Wait, you're not re-releasing your album. You're putting it out for the first time!"

Even so, I'm a fan of bonus tracks, and I leave the choice for you, whether or not you want to check out the early and alternate versions I'm including here online, as both curiosities and added value.