To reach the "correct" ruling (the sturdy house) so that a person knows exactly what their rights and responsibilities are before God.
Like a master's blueprint, it is the methodology and set of tools (logic, language rules, and historical context) used to understand the divine texts.
For a deeper dive into this academic discipline, you can find the complete text of "Fıkıh Usulünün Mahiyeti ve Gayesi" published by the İSAM (Center for Islamic Studies) . USUL AL-FIQH: ITS EPISTEMOLOGY, PURPOSE, AND USE
One person looked at a single line of the text and immediately started laying bricks. However, his house collapsed because he didn't realize that other pages in the manual provided rules for the foundation and climate. This is like trying to practice Fiqh without the methodology (Usul) .