Do you need gasket glue to change a valve cover gasket or is the new gasket enough itself Why then does gasket glue exist?

Answer 1

Sometimes yes, sometimes no. It depends on - A) what type of engine and B) what type of gasket. Older cars mainly used cork gaskets, so gasket sealant was a wise choice. New engines use rubber, some with metal bushings or spacers, so the typical "blue glue" as daddy used to call it isn't required.