Etienne has posted this elswhere viewtopic.php?f=12&t=17129&p=205054&hilit=reasons+for+preference#p205054
The box for reasons is only there because the law says you must have the opportunity to give reasons for your preference. It doesn't mean that they will affect the outcome or that anyone is going to read them when places are allocated! (I assume that there is a separate section for relevant criteria such as siblings, and that extra documentation is recommended for faith schools.)
Just one caution, though - if you happen to finish up at an appeal, it might look rather odd if you start giving all sorts of reasons for wanting a place at the school you're appealing for, but never mentioned any of them when applying!
Hope that answers your question.