What is the primary purpose of private IPv4 address space?

The primary purpose of private IPv4 address space (defined in RFC 1918) is to conserve globally unique, public IPv4 addresses. With the limited supply of public IPv4 addresses, private addresses allow organizations to use a large number of internal IP addresses without needing to register them globally. Network Address Translation (NAT) is then used at the network edge to translate these private addresses to a smaller pool of public addresses when communicating with the internet. While private addressing can simplify internal addressing and might indirectly reduce some aspects of network complexity, its fundamental role is resource conservation. Limiting nodes reachable via the Internet is a side effect of NAT, not the primary purpose of the private address space itself.