XML-RPC
XML-RPC is a remote procedure call (RPC) protocol that uses XML to encode its calls and HTTP as a transport mechanism. It allows software running on disparate operating systems and in different environments to make procedure calls over a network. The protocol is simple and lightweight, designed for easy implementation and interoperability between systems.
Developers should learn XML-RPC when building or integrating with legacy systems, APIs for content management systems like WordPress, or in scenarios requiring simple cross-platform communication without the complexity of newer protocols. It's particularly useful for quick prototyping, small-scale distributed applications, or when working with older web services that haven't migrated to REST or GraphQL.