When you are brand new to any sport, you begin by learning the fundamentals. These are the foundation for everything else you learn. If you think of a house with a poor foundation, then that is how someone would be who lacks or is limited in their fundamentals. The rest that is built will be on shaky ground. When you have a solid foundation then everything else built is solid as well and has them to fall back on.
In jiu-jitsu, learning and reviewing the fundamentals can not happen enough. Yes, if you are advanced, then you also want to learn advanced techniques. At the same time reviewing the fundamentals is always helpful to your game. They make everything more advanced in your game improve since the “foundation” is stronger.
Some can rush to want to get to the advanced technique, yet staying longer than planned with the fundamentals will have a much greater benefit to your game. Going back to the fundamentals will improve your game at any level. At the start of the advanced class, there are the same warmups that are done every time. Doing these repeatedly will not only help your game but put them in your muscle memory. The result is having to think less and less during your rolls over time.
Whenever there is a person that hits the same submission all of the time, it was repeated thousands of times. Any sub or sweep or position can become your go-to if you practice it enough. Looking at the fundamentals like that can help appreciate why they need to be continually practiced to become like a go-to sub, etc.
Everyone has different schedules and priorities when it comes to attending jiu-jitsu classes. If you are new then obviously you want to be going to as many fundamentals classes as possible. After 3 months to a year(depending on how often you train), you may try a mixed levels class. It is not until blue belt for most that it would be a good idea to go to an advanced class, while still attending fundamentals classes for the majority of your time. After time at blue belt and belts above, is it beneficial to mostly attend advanced classes?
The classes you choose are also important when it comes to rolling. If there is live rolling at the end of class, then you want one appropriate for your level. It is ideal to be able to work on offense with someone at a lower level, defense with someone at a higher level, and both with someone at your level. Sometimes you may have to go to a mix of classes to get all of these options. If you are just starting, then obviously you don’t have all of those options. It is good to keep them in mind though for when it might change in the future.
I don’t feel like it is that common, but you also don’t want to always be in the fundamentals when you are at an advanced level if your schedule allows for advanced classes. Even though fundamentals are always important, your progress will be slower. To keep in mind is to not ever discount the fundamentals regardless of your level.