The value of money changes with time. Prices tend to rise, this is call inflation. This means that 1£ today is worth more than 1£ tomorrow.
So what? Well if you are going to have something done for you in a years time. Paying at the beginning of the work or at the end is not the same. Paying in the future is “cheaper”.
This is one of the reasons why when you pay upfront you should expect a discount.