What are they? Check out the for-dummies, but what you need to know is that you communicate the details of beneficiaries to the institutions you want to warn (banks for example?) so that they will automatically transfer ownership of your assets to a designated beneficiary upon reception of proof of death.
Why are they important? Conflicts. If information between your will and the institution differ, the bank will prevail. It means your will can not be sufficient to ensure the proper transfer of ownership of funds in all the cases.. you catch my drift.