Doodledog's already said it: questions're not part of the selection, they're checking the life chances profile of those applying for their jobs, and that allows them to see if there are particular sections of society that are applying for a lot of their jobs but not getting the expected ratio of job offers. If monitored properly, a big business can then check that they are not, consciously or unconsciously, discriminating during the selection process & thereby not always getting the best person for the job,
The theory works better than the practice - but if the applicant doesn't want to answer the questions they can tick"prefer not to say". (or leave blank). The selection panel will never see the screening questions, they are retained - anonymously I believe - in the HR data so that the business has access to them. For instance if they were facing accusations of institutional racism & had statistics that showed a higher rate of successful applications from BAME candidates, they could use that as a positive to reassure current staff, the public & potential applicants.