Why do we need to set the request body as a UTF-8 string when testing the POST method in mockmvc

I would like to know how to test POST method so I am reading this article and this test looks like this:

@Test
public void createEmployeeAPI() throws Exception 
{
  mvc.perform( MockMvcRequestBuilders
      .post("/employees")
      .content(asJsonString(new EmployeeVO(null, "firstName4", "lastName4", "email4@mail.com")))
      .contentType(MediaType.APPLICATION_JSON)
      .accept(MediaType.APPLICATION_JSON))
      .andExpect(status().isCreated())
      .andExpect(MockMvcResultMatchers.jsonPath("$.employeeId").exists());
}
 
public static String asJsonString(final Object obj) {
    try {
        return new ObjectMapper().writeValueAsString(obj);
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}

I don't understand what this line of code is for and what exactly it does: .content(asJsonString(new EmployeeVO(null, "firstName4", "lastName4", "email4@mail.com")))

content(String content) from the documentation:

"Set the request body as a UTF-8 String. If content is provided and contentType(MediaType) is set to application/x-www-form-urlencoded, the content will be parsed and used to populate the request parameters map."