I think the point is that education is done in levels. There may only be a few people who use specific mathematical skills, but you wouldn't be able to go directly from GCSE level to Degree level without understanding what is taught at 'A' Level. Many 'A' Level student might not use what they have learned, but it does prove a level of knowledge, understanding and ability that employers look for.
For those who go onto Degree level, the Degree is likely to be more specialised and hence will concentrate on specific areas of maths with a view to qualifying someone for a specific career or group of careers. The knowledge acquired at 'A' Level maths, shows that a student has the potential to possibly complete the Degree course as well as providing a suitable foundation for the course.
As far as use of the knowledge is concerned. The understanding is essential in order to apply the knowledge and develop it further. A computer calculating something is not necessarily going to enable you to know whether you can improve what you have done or even whether you have entered the data correctly.