Because each shop only sold one general type of ware, shops needed to attract customers who might otherwise just be passing by. So prices were displayed on the goods in the windows.
Similarly once customers were inside buying one thing, the shopkeepers needed to attract them to buy other things by putting the prices on their counter goods.
Price-display cards were quite large - perhaps of a size between that of a child's and an adult's hand - and the currency was the pre-1971 non-decimal pounds-shillings-and pence system. The following photographs show samples of price-display cards as they appeared in the early to mid 1900s.They were taken in a range of museums.
See the separate page on how these prices would have been pronounced.