You've worked with SOLIDWORKS PDM for years. You're fluent, you use it every day, and you know your way around the program. So why would you need continued training?
The most basic example I can think of as to why you have to keep training is fitness. If you've worked for months or even years to get into the best shape of your life, then you're there and you decide "okay, I don't need to work out anymore." You won't stay in that shape, right? Of course not! Fitness, like a lot of things in life, is what we like to call a "use it or lose it" skill. As is pretty much all math!
My first job was at QuikTrip. I was trained by someone who was specifically selected to train clerks, and I learned a lot in that week of training. I learned how to do things by the book. After my week was over, I got sent to the store I'd be working at, and continued learning how to do things around the store. At my store, I learned from other clerks and managers. Not people who were selected to train, but still very knowledgeable. Two years later, I was still re-learning things that the clerks I worked with taught me incorrectly or inefficiently.
Now, these clerks didn't intentionally mislead me. They just taught me what they knew, which happened to be wrong. Or they taught me what they knew, adjusted to how they thought it should be done.. which was still wrong. Some of these things were valid tips, I learned how to count change faster and more accurately than most people, and I can power wash a parking lot like nobody's business. But guys, I was cleaning the coffee machine wrong every time I did it for two years. Don't tell Chet, but I didn't know! It's just how I was taught! Tim calls this "tribal knowledge." It can be useful, and even become a trademark of the business, like Chic-Fil-A's employees saying "my pleasure!" But it can also be pretty damaging and really hard to get a handle on sometimes.
Are you cleaning your metaphorical coffee machine wrong? What if there's something that you're doing, but not doing well? Wouldn't you want to know? Just because you drive the same route to work every morning doesn't mean there isn't a better way. If the state built a new freeway that reduced your commute, you would adjust your route to do things the faster way without hesitation. SOLIDWORKS releases updates all the time, they build those freeways in the program you use daily. I would be willing to bet, you have a coffee machine, or a new highway, out there that you don't know about.
Training isn't supposed to teach you what you already know. It's supposed to help challenge your understanding, provide new insight and information, and make you better at the topic at hand. Regular training actually has a litany of benefits for everyone involved. Almost all of our own clients experience a little bit of backpedaling after their integrations. Everything runs smoothly and designers use the tools we implemented for a while, then people just go back to their old habits. Things start to get messy again and without regular training, it can turn into a whole disaster before we get it back on track.