That's really an extremely broad question to answer. So let's break it down to bits then answer it.
The exam preparation teaches you how to manage your time, how to be thorough with your concepts and how to handle trivial questions that may come in exam. The preparation also helps you get regular in the studies as it is extrememly required when you join a management institution.

CAT the exam itself judges you on your problem solving, quick thinking, and time management skills. The subject wise knowledge is tested as well but the first three matter more to complete the exam.

When it comes to results CAT score by far is one of the most accepted score when it comes to admission to management institution. NOT only IIMs various other colleges that you can find in this link:

https://bschool.careers360.com/colleges/ranking/2016/top-mba-colleges-in-India-accepting-cat-score

Cracking CAT exam also opens up your horizon to various opportunities and challenges as well.