A guardian for children needs to be nominated in your will.
You may die failing to have transferred significant assets to your revocable living trust.
You may be killed in an accident caused by another and your named executor will need to open a probate proceeding to take action against the wrongdoer.
You may be the beneficiary of another's estate and then die before the other person's estate is distributed to you. Your executor will need to take action to acquire those assets.