Why does Ralph feel it's important to talk about their fears at the assembly?

Ralph wants to get a hold over the chaos. Par of the chaos comes from fears of the unknown. The "Littluns" are terrified of the Snake Thing and this fear filters to the "Bigguns" as well. Jack is taking advantage of the fear. He is able to manipulate these boys by providing safety and perceived protection. Ralph tries to eliminate these fears by sharing them and then logically discounting each one. Unfortunately Ralph can't legislate fear away. Ralph's attempts to rationalize fear crumble. The boys are still children with imaginations and fears of the unknown. Simply voting to not be scared does not work and Jack is able to manoeuvre this to his advantage.