You want to see why most people would stand to benefit from training classes? Just observe them at a training class.
Then, there's that group who proudly attend one or more training classes, but somehow don't bring back anything of discernible value when it comes time to perform.
In earlier years of martial arts training there were those who practiced to look good in front of the wall mirrors, and those who practiced to be able to draw upon and execute their training under stress and duress.
Training has to be a continuous process. ...
This ^^^ is the core of it, to put it as simply as possible. Once I was accepted in our FTU as a newly minted 'firearms instructor', I quickly realized it was going to be an apprenticeship program, and a continuous process.
In a way, it's like being a student in one of the belt-ranked systems who reaches the advanced beginner stage of achieving first degree black. You've worked hard to reach a point where you have an adequate command of the basics, and you're looking to continue to learning more advanced skills ... while now knowing that it's continually working toward mastery of the basics that's going to remain important.
As a trainer, though, now you have to be able to unlock and understand the ways to pass your knowledge and training on to others. In LE training nowadays, you can find a training program called Instructor Development. While it's not specifically tailored to firearms training outside the classroom environment, the core principles of understanding adult learning methods is still helpful.
Dunno what they're doing in the last few years, but I attended a LE Firearms Instructor Update class before I retired, I was appalled at the lackluster skills demonstrated by a dismaying number of 'working' firearms instructors from various agencies. Then again, nowadays you can find people assigned to be agency firearms instructors after they're sent off to a basic instructor class ... and then 'they are one'. I rather miss the days when an agency had the ability to have some vetted instructor talent in-residence, and treated newly minted instructors as apprentices for the first 2-3 years. Guess that was one of the benefits of working for an agency of roughly medium size.
Bottom line, training remains a continuous process for some years. Depends what you hope to achieve in the way of training and abilities, though.