For some, they buy it for its intended purpose, which is to be able to crack the window a few inches in inclement weather without getting your interior dirty. Most get it because they feel it makes their car look better.
I never got on board with that look, but then again, I also dont tint either.
I like the way they look but more importantly their function. I tend to crack my windows a lot to let fresh air in, especially when it's raining. I do not smoke/vape, etc. Fresh air circulating in the cabin while decreasing window noise is a win for me.