In IOT, CoAP is used as a Binding protocol for oBIX interfaces. The oBIX specification provides for bindings using REST (using HTTP or CoAP), SOAP, and WebSocket to talk with web/internet. In many cases, your choice of binding may be dictated by the application needs, such as if you are implementing a server in a constrained environment, or for an enterprise level application.
For CoAP binding following CoAP intefaces are mapped with oBIX requests:
OBIX Request - - HTTP Interfaces
Read - - - - - - - - - GET
Write - - - - - - - - - PUT
Invoke - - - - - - - - POST
Delete - - - - - - - - DELETE