Good reasons for virtual currencies.
I like being able to shop globally. I dislike rampant consumerism, but for the quite particular things which I decide that I will buy, local availability is often non-existent. Therefore, I need something other than cash or check -- something which allows digital transactions.
A credit card, paid off every month, works fine as an online proxy for cash. Except that people who work hard and diligently at crime would like to be able to ransack the value of my credit card. So there's an evolutionary arms race between users of virtual money and thieves pursuing same. (OK, all money is "virtual" in the sense that it is a stand-in for goods and services of value. That's already been brilliantly covered by several commentards.)
Alternate currencies, more explicitly digital, might attempt to address the issue of security -- security both from criminals and from quasi-criminals like banks and advertising agencies*. But so far, I don't see anything that is in a practical sense much more secure than the sensible (careful, cagey) use of a bank card. Bitcoin is in a theoretical sense more secure, and especially more private, but given the ability of dodgy apps and malware to ransack digital wallets, the cryptos can be at least as vulnerable to theft as bank card identity.
I'd welcome a well-planned, cleverly and securely implemented digital currency backed by a genuinely disinterested organization -- an org similar to the Free Software Foundation, for instance.
Accepting a digital currency managed by Facebook would be like sending your doped-up teenager to a drug detox center managed by El Mencho. It would be the worst possible idea.
*Personally, I think the internet advertising industry is built on bubbles and candy floss. Really. Huge amounts of money are spent and huge troves of personal data are collected in order to sell toothpaste and soft drinks? Someday someone will notice that online ads yield a spectacularly low return on investment. Or maybe that's just me -- I block online ads, I don't watch TV or listen to commercial radio (so no audio-visual ads there), and about the only ads I see are printed in the magazines or papers I read. And these are easily ignored. Therefore my user case may lead me to overestimate the uselessness of advertising. *shrug*