ResponseEntity trong Spring MVC dùng để làm gì?

@PostMapping
ResponseEntity<User> create(@RequestBody @Valid CreateUserRequest req) {
  User user = userService.create(req);
  return ResponseEntity.created(URI.create("/api/users/" + user.getId())).body(user);  // 201 + Location
}

@GetMapping("/{id}")
ResponseEntity<User> get(@PathVariable Long id) {
  return userService.findById(id)
    .map(ResponseEntity::ok)                     // 200 OK
    .orElse(ResponseEntity.notFound().build());  // 404
}

@DeleteMapping("/{id}")
ResponseEntity<Void> delete(@PathVariable Long id) {
  userService.delete(id);
  return ResponseEntity.noContent().build();     // 204
}