In my opinion -- there are two basic types of seminars.
One type is an "exposure" class. It's a chance to see something that you haven't done, or don't do often, or just get a flavor of it. It might be firehose experience with a lot of material dumped out for the attendees to try to pick up, but it's not really about the in-depth teaching. You might learn a complex form -- but not the drills that lead into it. You might get a taste of the art -- but you aren't learning it in depth. It might be a glimpse of something to come down the road when you let a beginner attend a seminar that's really beyond their current skill. In all of these, the main point is that you're just getting the idea of it... Maybe learning a particular drill or form. You may go home and practice some of it -- but it's not really going to impact your training deeply. It's a treat -- not a diet change. If we make a quick analogy to car mechanics -- it's really the quickie class in how to check your oil and change the tires on your new car.
The other is a deeper dive. It's what happens when you bring in an expert to teach some element in depth, or the point is to really shake up your training with a new approach and attitude. It may be broad -- but it's designed for you to go home, and keep working and developing. You'll be taught a drill and the application and how it fits into the system. If it's a broad exposure, it's going to shake up how you train, where you emphasize things. Instead of a treat, it's a reshape of the diet. Or, for a car analogy, it's a brake system certification.
Exposures are fun, like a trip to an amusement park -- but most of us don't want to ride a rollercoaster to work everyday.
Where it gets hairy is when someone attends an exposure, and claims that it gives them deep expertise. And, to make it even more frustrating, there are plenty of folks out there happy to collect money for a certification seminar that really doesn't go beyond exposure and giving out signed certificates in the grip & grin photo. (Note that there are certification seminars which build on lengthy work and actually test the material at the seminar -- not instantly grant license and "expertise"; I have no problem with those, they're just fewer and harder to find.)