Private lessons can often be under looked as a method to improve or maximize ones training. In any field of study, to truly get better its the ones who seek private instruction, and or tutoring who undeniably stand out from the rest. For students for example, in a group music classes for instances, who supplement their classes with private sessions are typically the students who have a deeper understanding of their musical instrument or eventually become masters at their craft. 
For new students taking Jiu-Jitsu classes, close supervision of an instructor overseeing their development can truly shorten learning curve issues of grasping key concepts during normal group classes. In typical classes, an instructors goal is to ensure that the average student is grasping the information being taught at a general pace that most can keep up with. For some who are a bit more in the upper level, an average level class may feel at times that it's easy for them. For the lower group, they may be struggling to grasp an average paced class, that may leave them feeling lost or overwhelmed or simply frustrated. So clearly additional needs are needed in both the upper and lower level students. To meet the needs of the upper student, private lessons can help supplement them transitioning into the next level or by helping them prepare for higher level up coming competitions. For those in the lower levels, their needs can be met by drastically slowing the pace of how a particular technique is broken down, even better explanation of key indicators of when a particular move should be done, such as learning to feel the weight distribution on certain positions, avoiding telegraphing or how to better control their breathing. In addition, the lower level student may simply need help catching up so they don't feel lost in a regular group class or even within a fundamental class.
For the average student who are in a group classes already tailored for them they too can still benefit from private lessons. Sometimes these students can feel like they are reaching a plateau or find themselves repeating the same mistakes over and over again during regular training sessions.
In some cases work schedules may at times interfere with the students progress and sometimes students can end up feeling discouraged. Private lessons can remedy this issue by helping them fine tune their previously learned techniques or help them fill in those gaps of lost training time.
So there are truly lots of benefits from private lessons that can remedy the difficulty of understanding key concepts, fixing repeated mistakes, improving movement patterns or simply to cater to their individual needs. Some of the most successful students at some point have sought out private instruction. When you think about it the insights and details in a single private lesson gained is the equivalent of one months worth of group classes. This is mind blowing to realize how much depth one can obtain and comprehend within a single hour private class.
Its a missing link worth looking into.