Accountability and being in control of your training will see you make huge progressions on your training as opposed to just showing up and winging sessions, the reason for this is because you are in a position where you can manage all variables such as your nutrition and training and understand how to make more progress.
Tracking your sessions in particular is very important and so simple to do, using a log book will ensure you are keeping track of how much you are lifting and exactly how much progress you are making. Plus when applying progressive overload it can be hard to see how much you are lifting over a long period of time by using a log book you are able to look back and track how much and what exactly you are doing on a session to session basis.
Structure in your training will provide a more stable base to progress from by tracking your training you are simply providing more structure in your training therefore allowing yourself a better oppurtunity to progress from. Other than a log book there are plenty of other ways to track your training for example you can use video feedback and analysis to measure how you are performing certain movements and over a set period of time you can aim to progress how you perform the movement. You can also track how a movement feels internally and push to make sure that you feel the movement better each time and ultimately make the movement always works for you and not against you.
So you see there are plenty of ways and reasons why you should be tracking your workouts and from those points above as long as you follow those then you cans assure you'll be making plenty of progress!