HttpOnly Cookie matters because security failures usually come from trusting input, leaking secrets, weakening permissions, or skipping verification. It gives builders a precise word for the thing they are changing, debugging, reviewing, or shipping.
ELI5
Think of HttpOnly Cookie as a lock, rule, or alarm for the app. It gives one small job a clear name so the whole app is easier to understand.
In practice
Use it when data, auth, payments, user-generated content, or external tools touch the system. In practice, name the owner, input, output, failure mode, and proof before relying on HttpOnly Cookie.