Numerous reasons (as there are numerous people)

: adherence to their own self-imposed precautionary principle
: adherence to their superior's precautionary principle
: they are showing systems
: they were in significant contact with other(s) tested positive (real or not)
: lemmings
: think it's the law (maybe it is, dunno)
: think the test results mean something, e.g if tested positive the for sure "I got it."
: fear of death
: fear of illness
: fear of breaking out of the pack's norms
: etc. etc. etc.

I simply can't understand why hundreds of thousands of people are getting tested every day. What is their motivation?

My son was in Benidorm when the government unexpectedly introduced the quarantine period.

He had the choice of two weeks at home without pay or getting a test to enable him to get back to work.

I also know of three professionals (a doctor, a dental nurse and an anaesthetist) whose kids got sent home from (different) schools because of a positive/suspected case at each school. Again, they needed to get tests to enable them to get back to work. In the case of the anaesthetist, all the operations she was due to assist with were cancelled (probably the people waiting for operations also had to be tested before they were allowed into the hospital).

I suspect most of the asymptomatic people being tested have a practical reason such as the ones I've mentioned.

I certainly wouldn't get a test unless I had symptoms (and maybe not even then), but then again I don't require a negative test result to enable me to get on with anything important in my life. If I did, I suppose I'd have to get tested.

There's an interview with the Abertay student who tested positive in the Telegraph today. 500 students from all over the country had just moved into an accommodation block together. This student had symptoms, and I suppose he didn't want to start his first week at university being responsible for some of his fellow students being ill.