(non-US, but don't think it's relevant to the answer. Other countries have similar scenarios)
It's not specified how the entrance exam exactly worked - if the exam + maybe other inputs are used for a ranking in which the best are picked, the group forming the top X students in that ranking can be quite different from X students randomly picked from the Y students with a good enough GPA (Y is substantially larger than X).
In addition to that, an exam can prioritize things through its design. The obvious one is fields (a school can focus on STEM, or language skills, or ...). But it can also target other skills: a simple example would be stacking it with questions that require more cross-discipline thinking, or quickly working with previously unknown things, which normal school exams don't tend to particularly test for.
And it has a chance of picking up people who fit the profile for the school, but whose GPA is not good enough because they struggle with some subjects or with their current school - people can be gifted but do quite badly in traditional schooling. An extra entrance process at least gives them a chance to jump past that.
Random choice from "good enough" GPA just gives you a random set of generally good students. If you have a focus in mind for your school, you probably can create something more extraordinary by stricter targeting (although that's not without its problems and challenges!)
It's not specified how the entrance exam exactly worked - if the exam + maybe other inputs are used for a ranking in which the best are picked, the group forming the top X students in that ranking can be quite different from X students randomly picked from the Y students with a good enough GPA (Y is substantially larger than X).
In addition to that, an exam can prioritize things through its design. The obvious one is fields (a school can focus on STEM, or language skills, or ...). But it can also target other skills: a simple example would be stacking it with questions that require more cross-discipline thinking, or quickly working with previously unknown things, which normal school exams don't tend to particularly test for.
And it has a chance of picking up people who fit the profile for the school, but whose GPA is not good enough because they struggle with some subjects or with their current school - people can be gifted but do quite badly in traditional schooling. An extra entrance process at least gives them a chance to jump past that.
Random choice from "good enough" GPA just gives you a random set of generally good students. If you have a focus in mind for your school, you probably can create something more extraordinary by stricter targeting (although that's not without its problems and challenges!)