Every ghost-chasing group of teenagers has to have the pretty girl who falls through the trap doors or gets taken hostage by a glow-in-the-dark zombie covered in seaweed. Daphne's role in the group has often been a mystery all its own until later stories pointed out that she comes from an incredibly wealthy family, therefore making it clear that she's the financier of the operation.
There is also a theory out there that she is actually Fred's girlfriend and he had to bring her along.