My sister recently had an interview with Sainsburys, and told me what it was like.
They did ask, “Why do you want to work for Sainsburys?” and she was able to come up with a unique, different answer that they liked. As with most employers, they are interviewing hundreds of people, and the important thing is that you can stand out and make them believe that you are the right person for the job position.
Avoid answers like “It looks good to work here” or “I like the pay rate” or “I need a job”. Also avoid stealing other people’s answers and answers that everybody tends to repeat.