You have the primary responsibility for ensuring the well-being and care of your children. If something happens to you while they are minors, you want to ensure you've made the decision about who cares for them, and how.
In the most ideal scenario, your child's other parent would be suitable to take custody of your child, if you cannot be there. But in many cases, that's not possible or desired.
And, even if it is, you may want the financial resources you are leaving behind cared for by someone other than your former spouse or partner.
No matter what the scenario, as a single parent (whether your child's other parent is in the picture or not), you need to take the steps necessary to legally document who you would want raising your child, how you would want your child raised, and how you want your assets handled for your child, in case anything happens to you.