Juried shows are a place to exhibit work that expresses my vision as a photographer. The positive feedback of having a piece accepted into a show judged by someone with an art education and background reinforces my internal drive to keep producing work.
As far as entering the same pieces in multiple competitions, our local art council sponsored 2 contests at the same time this summer, although at two different venues. I emailed to ask if I could submit the same two photographs to both contests, and they encouraged me to do so. One of the imaged declined from one show was accepted into the other, so as DW commented, it is a crap shoot completely at the subjective whim of the judge.