Usually once you pay for something, you own it, and the transaction is complete. That’s how it works with, for example, sneakers, pretzels or dog shampoo.
But in technology, you’re never really finished paying. Buying Microsoft Word, or Quicken, or an iPhone may feel like a one-time transaction, but it’s not. Every year you’ll be offered the chance to buy a newer, updated version.
Since Word debuted in the 1980s, upgraded versions have been offered 14 times. Since Photoshop 1.0 in 1990: 20 times. If you’d bought Photoshop in 1990 and stayed current by buying each annual upgrade, you’d have paid more than $4,000 by now. 
 By David Pogue for Scientific American