For example, imagine being part of a security team of an old castle.
Your team would need to decide how to allocate resources to defenses.
Giant walls, stone towers, and wooden gates are a few common security controls of these structures.
While these are all designed to protect the assets inside from attacks, they don't exactly account for all the possibilities.
What if the castle were near the ocean?
If it were, these defenses would be vulnerable to long range attacks by ship.
A proper understanding of the attack surface would mean your security team equipped the castle with catapults that could deal with these kinds of threats.