There are quite a lot of uses for a thin metal knitting needle when rigid heddle weaving, I think. From clearing a sticky shed to counting picks and even (I know I shouldn’t) poking my fell line, I’m always glad to have one on-hand.
Of course for those uses it doesn’t have to be metal, but I like a metal needle because it has a certain weight to it. Also, I tend to use it to provide lift-assistance to my string heddles and a metal needle never feels like it’ll snap under tension!
As with most such tools though, I probably find more uses for it simply because it is right there to be used!