The trap prevents carbon dioxide from entering the atmosphere over the reaction. The trap captures HCl fumes that are evolved from the reaction mixture. The trap captures sulfur dioxide fumes that are evolved from the reaction mixture. Using the trap gives the student experience with a reaction conducted under a controlled atmosphere. The moisture would form sulfuric acid from the evolved sulfur dioxide.
Water slows the reaction between butadiene and maleic anhydride.
Water hydrolyzes maleic anhydride to maleic acid which can isomerize to the unreactive fumaric acid. Water hydrolyzes maleic anhydride to maleic acid which reacts very slowly as a dienophile.