Whats the reason for nudging someone near you when someone walks by?

I can name different instances when people have done this and usually it's negative. It's usually to point out something about someone to laugh or when someone thinks they're better than you because of their race or jealousy or for whatever reason.

What are reasons you would do it though or that you've known others to do so? Especially in a situation where there's guys and a girl walks by.
Whats the reason for nudging someone near you when someone walks by?
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