@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.