In 1999 Amazon applied for and was granted a US Patent for One Click Purchase. Before then, everyone had a shopping cart that you had to go into to check out and pay. Amazon realized that a huge percentage of people would add stuff to their cart and then leave without buying anything, either because they decided they didn't REALLY need that thing or because they found it cheaper somewhere else or whatever. They allowed you to save all your credit card info plus shipping preferences, then just hit "1 Click Purchase." It was convenient for shoppers because they didn't have to go through the whole checkout steps or add everything then come back later to check out. They could just hit a button and be done. For Amazon, though, it prevented the dreaded "items left in cart."

Other sites like Borders and Barnes & Noble, etc also implemented the feature, since it made a lot of money. Amazon filed for a business process patent (I think they also tried it as a software patent??) and forced the entire internet to go back to normal shopping cart purchases. They ended up losing the patent lawsuit in the EU, but that didn't stop them from enforcing it on US websites. Borders and BN both implemented "2 Click Purchase" to get around it, but the damage was done. In everyone's minds, Amazon was the place to go for convenience and speed. Amazon made more money, while others started losing money. With that extra money, Amazon was able to move into the "niche" of Walmart, since Walmart hadn't yet figured out e-commerce. Amazon out-Walmarted Walmart on the web and became the trillion dollar behemoth we have today.