The body, json body and xml body options, present in the assertion options and the store options (except for body) of API actions, all have the same goal of helping you run precise assertions on API responses.
Since an API may return responses in different formats, we need to specify which format to expect, so that the test runner is able to parse the response correctly. Then, we can run assertions on the data extracted from the parsed response, or save it in a variable for later use.
The json body and xml body options allow us to run more fine-grained operations on the response, such as retrieving deeply nested information, while the body option for assertions only allows basic checks such as length check and text comparisons.