"Harry is mindful that Tom Riddle has been killed before," said Neville. "His Horcruxes keep his spirit anchored here. We wanted to make sure that when the final battle came, we destroyed his body and spirit. Harry could easily kill Riddle in a duel, but his spirit cant be allowed to escape again, maybe possess someone like it did with a Hogwarts Professor we had in our first year. So we wanted a route to the other side, where we might find help. Others on that side who could drag him through or trap him once he got there. There's only one place we know where such a doorway exists."