In most cases it’s the details that get left behind when writing a will.
You may note that you want your home to be paid off and placed in a trust, but you may forget to list who you would like to live in it, or even who manages the property if't is to be rented out.
There’s also the issue of listing only one guardian for your children, or listing a couple and joint guardians but not noting whether this would change if the couple gets divorced or passes away before you.
Valuable material possessions are another issue to consider. If you have items of great value you may wish to leave them to specific friends of family members to avoid arguments.