Separation Agreement vs Divorce Decree

I have never quite understood something and was hoping for clarification.

Signed separation agreement regarding multiple things, but child support is what I questions. The separation agreement was not incorporated into the Divorce Decree. Can one not hold the other party to the agreed upon child support amount if the court later modifies? If so, then what is the purpose of a separation agreement, really?

What about items that the Court cannot order, such as college expenses, etc?

The court does not have the authority to modify the provisions of the agreement, but can enter a different child support order. If the court enters a new order, either party can still attempt to enforce the provisions of the separation agreement using a breach of contract action. Under North Carolina case law, the provisions of a separation agreement are presumed just and reasonable unless a party can rebut that presumption based on evidence of a significant change in the child’s reasonable needs.

A separation agreement also allows you to contract for provisions that a court doesn’t have the authority to order under the statute, like college expenses.