It's so that the compilers/judges do not have to do the boring job of checking all your answers, they only have to look at one number and they also charge you £1 if you text the answer so they win all round
It is an added part of the crossword as mentioned saves compiler from checking answers it is a bit of a chore as we always have to double check, but I enjoy crosswords and if you buy the paper for that reason you might as well finish it and post it. I did have a win on this crossword a long time ago when the prizes were smaller still waiting for a big win!!