Another nice thing about structured classes or scheduled lessons is that it holds you accountable, lays out a roadmap, and creates deadlines.
Life has a habit of getting in the way, and even the best intentions of "I will practice by myself 15 minutes twice a day, and practice in public places every Saturday!" falls apart when work, family, house, car, etc. problems rear their head.
It's just the way life goes. Happens to all of us.
When I know
I have a scheduled class/lesson coming up, I force myself to make time to train and train some more beforehand, so I've visibly made progress over the previous week.
Otherwise one week quickly blurs into the next. I do better with benchmarks and check-ins, and I know I'm not the only one!
Good trainers also hold you accountable for whatever it is you're supposed to be working on - whether you're pushing hard to hit a competition goal, or simply aiming for solid reliable obedient behaviors.
Another unrelated (invaluable) benefit is growing your dog network. The people you meet at training end up becoming resources, carpool allies, trial buddies, help if you have a dog emergency, and in the best cases - friends.