People really underestimate how much of an impact this has. Here's a completely made up example. Let's say a person of a family of 4 goes grocery shopping once per week. This person buys 30 grocery items on each shopping trip. The difference between Wal-Mart and the local place might only be $0.25 per item. Is it really worth it to go somewhere else to save $0.25? No, unless you're saving $0.25 on everything you buy.
At $0.25 per item with 30 items each week and 52 weeks in a year, this shopper saves $390/year or a little more than $32/month by going to Wal-Mart. $32/month is pretty meaningless to most people who command a developer's salary, but there is a sizable portion of people in the US who could really use an extra $32 each month. That's the difference between Wal-Mart compared to the local shop, and that's the reason why Wal-Mart edged these small stores out of business.
My family actually drives past regular stores because of the selection at a super center.
They sell 1/2 a trillion worth of stuff. They have a bigger economic impact on America than google, Facebook,Apple etc.