Why do TV crime shows constantly decide whether a suspect is left or right handed by which wrist they wear their watch on?

I see this all the time on shows like CSI and NCIS, they say a suspect must be left handed because he wears his watch on his right wrist, and they can eliminate suspects that way. That seems like a very wishy-washy way of investigating, do real world detectives use the same method or is it just a TV thing? I'm... show more I see this all the time on shows like CSI and NCIS, they say a suspect must be left handed because he wears his watch on his right wrist, and they can eliminate suspects that way. That seems like a very wishy-washy way of investigating, do real world detectives use the same method or is it just a TV thing? I'm left handed and wear my watch usually on my left wrist about 90 percent of the time, its just a personal choice and depends on certain things like what pants I am wearing and what I have in the pockets. WTF?
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