The biggest reason to dual-box for me dates back to EverQuest, where you NEEDED another character (heal-bot) to help you out. The game was far from solo-friendly. Ever since then I have at least dual-boxed every single MMO I have had (EQ, EQ2, AC, AC2, E&B, SWG, AO, EVE and now WoW).

If you have the hardware lying around, why not do it?

People have already justified the costs of hardware, accounts and subscription fees. But if you're asking this question, then you'd probably ask the following:

1. Why do certain people pay $1,500 for a car, and then drop $30,000 in parts into it?
2. Why do certain people spend up to $10,000 for an A/V stereo receiver, and then up to $4,000 on each speaker of their 7.1 surround sound system (<- my Uncle).
3. Why do people spend anything above and beyond what is "needed"?

I'd say the first thing to come to mind is "bragging rights". Being able to show your friends your new "setup" will forever be a guy thing.

But with multi-boxing I think a lot of it has to do with the way MMOs are setup. Once you've been through the content 4 or 5 times it gets a little stale (I've literally brought characters up through the Barrens 9 times now -- not fun). End game is fun, but you find yourself wanting to branch out to other characters. So for me, I use multi-boxing as a way to bring multiple characters up to play in the end game together. It just gives me more options.

Ultimately though, the answer is: I have the ability to do it and I've already justified it to myself.

My rebuttal question would then be: Why do you care that I'm spending my money the way that I want to?