If there is no will, your children or other beneficiary will have to go to Court to get a ruling as to who gets what.  This can arise when there are blended families, like so many families today.
With a Last Will and Testament, you can state what you want, who you want to receive the things that you leave.  You may want to divide equally, you may want to give one child or person a bigger share because the other child or person already received their part.
Benefits:  You get to have your say in the Will.  And the Will does not start until you die, so you can change at any time.
The cost is not that much.