Speaking specifically of the kettles I have chosen over the years. I cannot speak for all Weber products obviously...
#!. Usability. Simply speaking, properly outfitted with the hinged grates, you can esaily set up, and maintain your cook with very little effort.
#2. Flexibilty. I have come from the world of larger dedicated offset smokers, and found that it is MUCH harder to go from grilling, to smoking on an offset than it is on a Weber.
#3. Durability. The ceramic based finish is MUCH better at resisting rust. I have had steel offset smokers rust out in less than 2 years. No such problem with my Webers. My Smokey Joe Silver is now over a decade old, and aside from replacing the cooking grate, it cleans up and works fantastic.
#4. Smaller footprint. I have 2 22" Webers, AND my Smokey Joe Silver in less space than my old New Braunfels Smoker , which means I can store my Webers, All of them, inside the garage workshop. They literally slide right in behind my lathe, once they are cooled off and clean of course.
#5. TCO. Total cost of ownership. Compared to the old offset smokers, for the same amount of food cooked, I use about 1/2 as much smoking wood / charcoal. The grills themselves were MUCH cheaper. My New Braunfels Smoker Bandera Smoker (same model now rebranded as Char Broil Oklahoma Joes Bandera) cost $350.00 back in 2006, My Jumbo Joe Premium, my Smokey Joe, and my old Craigslist find 3 wheeler Bar B Kettle has less than $160.00 wrapped up in. Add accessories and I am just barely over $200.00. Yes I could have spent more on one new top end model Weber kettle, but I wouldn't have any better results with my cooking...
I added both 22" kettles last year to give me the cooking space I wanted. Learning the techniques to get the best out of the Weber was super easy. My favorite results I can find pics for right now are from last week, I smoked a whole small turkey which we turned into 4 different meals...
And a couple of large pork loins that we got from the local store when they were having a sale on pork.
I stocked up on a mess of pork ribs at that time as well, all of which went together VERY well...