The title seems contrary but actually it’s not. It’s true.
Let’s start with analogy in civil engineering. A civil engineer knows better which column holds most load. Which one is the backbone and which one is the ornamental.
When a civil engineer has job to destroy building, he knows exactly which one is the critical column is. Techniques on how to destroy the column will follow.
This analogy applies also in software engineering. A software engineer who makes a web application will have better knowledge how to break web application.
What the framework being used, the programming language, hash algorithm, & so on. Techniques on how to break web application also will follow.
Even, if there’s no testing tool available out there, a builder will build his/her own tool.