There are a few reasons for the split. Most importantly a registry is tied to a TLD. A TLD can only have one registry for it. A registrar can work with multiple TLDs and multiple registrants.
Imagine them as filters.
A registry maps one TLD to multiple registrars.
A registrar maps multiple registries to multiple registrants.
Companies come with jurisdictions. A given company will operate, or be exposed to, one or more jurisdictions. In the modern Internet era this is even more important, but even before privacy became paramount you could never expect every registry to deal with customers around the world. There are different laws in different countries. So you need some local entity to act as a kind of middleman, if only to accept local currency and payment options.
This conversation has gotten more important recently because legal requirements around registrant data differ by jurisdiction. However, IMO this is simply a logical extension of the requirement for locality.
People around the world do business differently. Also, a registrant may wish to control domains under different TLDs with one account. That requires a registrar that can interface with multiple registries.
Imagine them as filters.
A registry maps one TLD to multiple registrars.
A registrar maps multiple registries to multiple registrants.
Companies come with jurisdictions. A given company will operate, or be exposed to, one or more jurisdictions. In the modern Internet era this is even more important, but even before privacy became paramount you could never expect every registry to deal with customers around the world. There are different laws in different countries. So you need some local entity to act as a kind of middleman, if only to accept local currency and payment options.
This conversation has gotten more important recently because legal requirements around registrant data differ by jurisdiction. However, IMO this is simply a logical extension of the requirement for locality.
People around the world do business differently. Also, a registrant may wish to control domains under different TLDs with one account. That requires a registrar that can interface with multiple registries.