Following are some of the points that will work for you if you join coaching. If none of these points resonate with you, then chances are that you do not need coaching:
1.
Speed up the on-boarding process: If you are starting cold, then chances are that you would spend significant amount of time to just get into the groove.
2.
Study Resources: For unsuspecting students who are self-preparing, chances are that they would burn their fingers a few times, before realizing the sanctity of authentic study-material for GMAT. If you join a coaching institute, then obviously the institute would ensure that you have firsthand access to the best study material.
3.
Instructors: Passionate and genuine Instructors of GMAT can make every dollar and every minute of yours, worth the experience. However, if self-learning is something that has always worked for you, then perhaps this is a point that would not appeal to you too much.
4.
Support outside the classes: Apart from the classes, coaching provides an invaluable support with concepts and doubt clarification all through your GMAT journey. In the current scenario, there actually are some good freely available support forums (such as GMATClub) available for students.
5.
Structured approach: The kind of material that you need to get access to, the sequence in which you should be studying the various concepts, and the best sources of those concepts are few pieces of the GMAT puzzle that coaching centers would have solved for you.
6.
Discipline and group-study: Classroom coaching, by its virtue of fixed timings, rigorous schedule, and predefined pace, would necessitate that you are disciplined in your preparation.
So, there you go, based upon the above points that come to my mind right now, do a self-introspection and decide accordingly.