@kaia they are, at least partially. Its just inertia, really, you can have kaia.de for free as long as you don't care about other people accessing it too.
@kaia pretty sure it's two parts, neither is required by IANA or anything...
- prevent conflicts of interest between customer and registry. you might not want to do direct business with Verisign if you are a competitor/enemy to Verisign. You'd pay Gandi who will pay Verisign on your behalf minus margin.
- separation and simplification of concerns, where the registrar is possibly an abstraction of many registries. You can do business with one total registrar but get many TLDs owned by any number of parties.
@kaia@brotka.st they often provide their own name servers. But that's an added service that's basically just expected for free now I think. All domain registrars do (as solely a registrar) is submit your info to the registry and collect money I think. Usually, the fees are pretty modest, but some companies like Endurance International Group like to charge very high fees (I've never heard something good about them).
@kaia i think it's the same season why you need to pay money do have a stand at the local Christmas market: people actually come there and you can serve them. You can sell your dream catchers and Peruvian flutes somewhere else too, but it just isn't worth it.
@kaia They don't maintain the DNS infrastructure as a whole (my domain has my own nameservers + slaves provided by my instance's UNIX-beard and Afraid.org).
But they do need to keep a decent bunch of infra going, which just costs money.