It's not hard to understand why people buy virtual goods if you map it to real-world "virtual values."

• Entertainment/Media - Real-World: a movie, roller-coaster ride. There is nothing tangible here except the physical boundaries it belongs to. Virtual goods can provide similar entertainment/media value.
• Social Status or Belonging - Real-World: a country club membership, new job title, PHD, Harvard, black belt. These elevate your social status, your "feel good" of owning them or belonging or the access it provides. Virtual goods can be mapped here - if you belong to a community where being part of the country club, or having a PHD is important to you; this is the analogous in the virtual world where it permits you with special access, community, or superior status.
• Brands/Perceived Value - Real-World: Nike, Porsche, etc. These physical items elevate your social status or express your identity, but also lets you self-affirm that you have achieved or arrived at some (usually financial) state because you can afford the finer things in life. In all practicality, a pair of Nike shoes is no better then a generic pair, and a Honda Civic certainly gets you from point A to point B (why a Porsche?)
• Utility, Shortcuts - Real-World: swiss army knife, insider knowledge, skip-to-the-front-of-the-line passes. As in the real world, we are willing to pay for access or knowledge to get ahead faster. Some of these virtual goods do the same within the game/environment context - a better sword, a boost, or tools to enhance the gameplay.
• Speculation, Flipping - Real-World: commodities, stock, real estate. Some virtual good items have real world value based on its demand. When you buy a contract/note for 1000 bushels of wheat on Chicago/Wall Street, you never really see the physical items either, but its value (whether its for hedging or to be resold at a different market conditions) work the same way in a virtual world. One can say,  the first virtual good in the real world may in fact be derivatives, stock, and currency (cash).
• Collectors - Real-World: collectors. Some people, well, collect real things. Stamps, Baseball Cards, Coke Cans, Pez Dispensers. These people do the same in the virtual world.
• Gifting - Real-World: flowers, thank-you cards, congrats messages. In the virtual world, people send virtual goods to express gratitude, intimacy and connection. As the adage goes, sometimes it's the thought that counts.
• Kids - Real-World: Parents buy all sorts of strange trinkets and toys: action figures, baseball cards, etc. Parents do the same with virtual goods for kids, this is what kids want today.

• Shorter answer (than reading the whole article): You buy virtual goods for almost the same reason you buy "real" goods: because they may benefit you. The only difference between virtual goods and "real" goods is that they don't have a significant physical component to them, but they may provide utility just the same.