This delay is the time difference between a router power
up and the time it can actually start preempting the
currently active router.
When a router first comes up, it doesn't have a complete
routing table. If it's configured to preempt, then it will
become the Active router, but it will not be able to
provide adequate routing services. The solution to this is
to allow for a configurable delay before the router
actually preempts the currently active router.