They are measuring effort, and effort is the closest thing an interviewer has to a predictor of whether you will accept the offer and still be there in a year. A candidate who has looked at the product, read the posting properly, and can name one thing that genuinely drew them in has already behaved like someone who wants this job rather than a job.
There is a second thing underneath it. Your answer tells them what you think they do. If you describe the company back to them and get it slightly wrong, or describe it in terms so general it could be any employer, they learn that you applied broadly and looked at nothing. That is why this answer cannot be written in advance and reused. It has to be built from their material, which is exactly what the tool above does.