The Graduate Management Admission Test also known as the GMAT is a critical step in the process for admission to graduate school.  Business schools are very competitive which gives them the option to select the highest qualified candidates for entry.  Your GMAT score is one of the largest determining factors to how qualified you are for their program which is why GMAT Test Prep is so important.

The average GMAT score is a 540 which is also at or below minimal GMAT scores for most business schools.  This means that Business schools are looking for above average, and in some cases for top business schools, significantly above average test scores ranging from 650 and above.   Most people that take the GMAT ultimately have to take it again because of the lack of GMAT Test Prep the pursued prior to taking the GMAT resulted in a sub-par performance.
 
This can be damaging because you can not erase or remove old scores.  All GMAT scores received will be available to business schools you apply to.   You want to do the best you possibly can on your first attempt.  

While you may be very intelligent and feel that you have a comfortable grasp on Algebra, Geometry, and some calculus, questions derived from these areas of math will be formatted into tricky questions designed to expose any weaknesses you may have.   Because of the adaptive capabilities of the exam, once you answer a couple of difficult questions wrong, you will be classified into a tier and fed questions within that level to confirm your skill level and ultimately your GMAT score.    Using GMAT Test Prep services to understand how to answer the difficult level questions and avoiding common pitfalls will allow you to stay in the top tier of questioning which allows for a higher GMAT score. 

This is just one example of how GMAT Test Prep courses, online courses, Private Tutoring or other GMAT Preparation will allow you to achieve your highest potential GMAT score and increase your chances of getting into your top choice Business School.