This is going to be a quick one, but many people don't understand why they have to open an estate just to cash a small check made out to someone who is deceased, close an account, etc.
The question usually goes like this "The will says I'm the only heir, why do I need a probate?"
At the time of a person's death a legal entity called "The Estate of (Insert Deceased's Name Here)". The estate, as we'll call it for short, owns everything that the deceased own the moment prior to their death. This can include their share of community property, separate property, a claim for a lawsuit in which they received the injuries that killed them, etc.
The most common reason for opening a probate is to pass title to land. In order to get title insurance and to get property financed there must be clear proof of everyone that owned the land and the transaction in which they obtained it. This is known as a "chain of title". Since the property is now owned by the estate, that ownership interest requires some document to pass title to the heirs and that is where probate comes in.
A will is really nothing more than an expression of what the decedent meant to happen to their property. However, the will is just a piece of paper with no legal effect until it is probated.
First off, just because a will says someone will be the executor it doesn't actually give that person any power. Only the court can actually give them executor powers when the will is probated. Anyone can write down anything and call it a will, but a will is required to meet specific guidelines to be valid and, in addition, the executor must qualify as well.
While the will may direct that the decedent desires to leave their house to someone, that "bequest" is made subject to any liens that exist as well as any bills that the deceased owed.
The same is true for any accounts, checks, etc.
To wrap this up a probate of some type is needed when you have to pass title to property, whether it be land, an account of some type, or something else where someone requires proof that the property belongs to a specific person.