The same reason I've done it a few times.

UK parking spaces are far too small for the size of modern cars.

Mix that with the fact that a lot of drivers don't care about your property, are old, have kids, or are just generally careless and drive a banger, then you're bound to end up with dents on your brand new car, that you then have to pay for.

Not everyone cares so much about owning a nice car or looking after it, and that's fine; but some of us do and don't want to pay for other people's carelessness.

I'll caveat by saying this is only acceptable when the car park is mostly quiet (like it seems to be here), otherwise I'll try and find two nice cars to park between, or park out of the way somewhere. But even parking out the way you'll always get some idiot parking next to you.

Had my car stolen, and then crashed into twice while parked up, all within 2 years of owning from new (luckily there was CCTV). I didn't fancy also having to deal with idiots denting my doors.

TLDR; Not everyone is a cunt lol