I also feel it's technically redundant -- like you said, they will follow up regardless -- but I think I do it to just give a more inviting tone/closing to the email.
However, I also used to end all of my StackOverflow questions with "Thanks in advance." just to be polite and appreciative to those spending their time reading/responding to my questions, until it kept getting edited out (for the same reason, redundant noise), so I got the hint and stopped.
Maybe this is similar?