My mate puts all sort of things like that. It covers his ... Because you can't see what's behind, like if your trying to cover something.
I'm sure there are more reasons. I would get an advisory for under tray being fitted but I take mine off come mot
As examples of the way it doesn't work - five years ago I had an advise on a rusty back box - never changed it as it wasn't leaking and its never been mentioned again. Last year I had an advise on the rear nearside tyre being close to the limit. With me working away most of the year I forgot to change it yet despite clocking up an extra 4000 miles the very same tyre wasn't mentioned at last weeks MOT (and its still legal!!).
Looking through my previous MOT if I'd followed their advice I'd be down three tyres, a back box and two lower wishbones in five years (they often advise slight play in ball joints).