Van Helsing has to track down and kill Dracula before he takes out Anna Valerious, who is the last remaining survivor in her family. If she dies then none of her family can get to heaven. Dracula also needs Frankenstein to awaken his babies and he uses the Wolfman to do his work.
Continuity mistake: Towards the end of the movie, in the scene where Anna is taking the werewolf cure to Van Helsing and the vampire bride stops her on the large fallen pillar, she falls so she is in a sitting position with the cure in her left hand. She then switches it to her right. Then in the close-up shot, the cure is still in her left hand and she switches it again. (01:49:50)