I think the main thing for me though, which no one has mentioned yet, is the idea of finding a new perspective. Each disc company designs its lineup of discs in a certain way to cover each part of the fairway or each type of disc/shot in a slightly different way. So the shot you might use a speed 7 fairway for in one manufacturer, you might be forced to use a speed 9 fairway for in another. It forces you to think about what you throw for different types of shots in a different way, which I think is an interesting exercise and often one that ends up changing how I do things in my main bag for the better.
When people are filling out one-brand bags and looking for replacements for key discs from their main bag, the advice I always give is not to look for a 1-to-1 comparison. Every mold is different, none will fly exactly the same as another.
It's always fun to find new ways to cover the full spectrum of shots needed.
Of course, it was this kind of attitude that got me banned from the Lat/WS/DD FB Group....