There’s a good thread on this topic on the Chalk Bucket forum.
I’ve coached a few girls that swear by them. The pressure seems to reduce pain below the knee. Not prevent further injury, merely reduce pain during training.

This is the most common kind used. But a couple of commenters recommend the double version as being more durable and staying on better during training.

Leave a comment if you have an opinion on these. I cannot recall seeing them on any other athletes than female Artistic gymnasts.
Updates from the comments:
Kecks tells that … “patella straps are used a lot in track (condition called “jumper’s knee”) and also in Olympic Weightlifting …”
Skyrider95 sees … “a lot of of male and female trampolinists and tumblers” … wearing them
Nica tells that … “figure skaters use them ALOT, we all have jumpers knees, we practically invented that injury haha” …
