There are many items, such as 401Ks; IRAs; stock and mutual fund accounts; and life insurance policies where you elect a beneficiary when you create the account.
These items when you die are considered to be outside your estate, your will, and your trust. The assets will go to the beneficiary selected when you established the asset.
This causes both planning problems and opportunities.
For example, it can cause a serious problem if you have a minor child named as the beneficiary. It can also lead too a bad result if you name a person to inherit the item expecting them to share it with others when there’s no guarantee that will happen.
Another example is if you set these accounts up when you’re married and later get divorced but you forget to change your beneficiary out of your ex’s name. Then your ex will be entitled to the items when you die. This is good for your ex, but likely not what you intended to have happen with those assets.