It's mostly for dynamic pricing (based on time of day, number of customers, weather, you name it). It'd be more expensive and time consuming to send an employee with a label printer all over the store a couple of times a day.
What happens if I put an item in my cart when the tag says it costs three shillings, but when I get to the register they want five?
The only time I've seen dynamic pricing was the KMart blue light special.