"What makes people think it's so necessary and so important for Britain to be part of a European union?"

Brainwashing with socialism, that's what.  Remember the hysteria  of Britain's militant  unions, and powerful socialists and  communist  intelligentsia against Thatcher, saying she would destroy  the nation, The truth is she saved Britain from a Soviet style bankruptcy and it is her legacy of economic security Briton's now wish to preserve.