There are so many threads on continuing education. There are also so many people that feel strongly about it, one way or another.
Currently there are no 'diving police'. If you don't want to take any classes after open water, don't.
There are also people that feel more comfortable diving with an instructor or want to possibly shorten the learning curve on a specific type of diving. I say possibly because it really depends on the instructor and the student and what each one wants out of the class. Granted some specialties have very short learning curves while others take a bit longer.
Once you have an open water certification card, you don't have to apply for another. The cards are just to show recognition that you completed a specialty. (Though there are some occasions where you may need to show a specific specialty card)
Some people, I’m one, aren’t satisfied with just learning the bare minimum. When I was an open water diver I wanted to expand my diving horizons so I took specialties. I see specialties as just introductions into different types of diving. There were some specialties that I really enjoyed and that I continue to build upon today.
Yep, you could go straight from open water to 130’ dive with a buddy without having to worry about the diving police. But is your buddy going to be doing that dive solely to monitor you? Is he/she going to be watching you closely to monitor for narcosis, keep an eye on your air pressure, and make sure you are going to do the safety stops or is he/she going to be diving to look for the cool stuff down at that depth? Are they going to talk about deep stops, emergency decompression stops, hang bottles, and rock-bottom, are they going to have a first aid kit and emergency O2 on hand and know how to use it? What happens if stuff hits the fan? Hopefully your buddy is competent enough to help you out if the need arises.
Sure there are certifications that aren’t for all divers, but heck some are just plain interesting. (While on Kona I REALLY wanted to take a Manta Ray Specialty. Would that make me a better diver? No. Did I need that to dive with the Mantas? No. Would I have learned more and possibly enjoyed diving with them more? I bet I would have. I just didn’t have the time to do it yet.)
Some of us may see some specialties as ‘fluff’ but for another it may just lower the anxiety of doing a new type of diving. Or it could just be a way to learn something new about the ocean and ourselves.