Because a probate makes it possible to transfer those assets, i.e., to sell them or distribute them to the decedent's heirs. With the decedent gone, no one else can sign the deed or other document transferring title. Only a personal representative has authority to do this. A personal representative is needed to transfer that share. An affidavit of surviving joint tenant is sufficient to prove ownership of the home.
Life insurance proceeds.
Personal possessions that are not titled. Without a probate, assets received by the heirs may be subject to those claims. With a probate, a notice is published giving the creditors three months in which to file claims. Unfiled claims are cut off. The decedent had a Will, and you want to be able to enforce the Will in court.