So apparently these shoes are on sale in Debenhams, and seeing the SAVE 50% in eye-catching bright red letters reminded me of a persuasion technique called 'anchoring'. Anchoring is fundamentally a technique that sets a standard or an 'anchor', and lead the audience to base their decisions on this standard.
So Debenhams decided to put some shoes on sale, but why would they state the original price? Or the percentage of how much you will save? The reason Debenhams went as far as to write 'SAVE 50%' and state the original price (and the price after!) is because they are trying to use cognitive bias to cloud consumer judgment. As they emphasise the fact that these shoes are on sale, consumers are more likely to place too much focus on the original price. This leads them to believe that these shoes are worth the amount stated before sale, although for all we know they were never worth their current price to begin with.
Anchoring is a truly useful technique if you are a marketer. Research has suggested that despite the fact that the level of anchoring effects can be reduced through experience and depends on cognitive ability, the effect was never eliminated completely.