RestController Response with JSON vs. Controller Response with Model

Question

I am studying sprint boot these days.And i'm trying develop a e-bank project with Spring boot. I can't understand why to could get direct response with JSON of restcontroller. I could use Model object with end-point that return of controller method. And i could process that model object to purpose on front-end side. But i don't know how to procces if i get response as JSON directly. If i could not, why could we return JSON directly? Can you explain these for me ?

You can take a look my restController and controller classes at my practice project

RestController response with JSON

@RequiredArgsConstructor(onConstructor = @__(@Autowired))
@RestController
@RequestMapping("/rest/customer")
public class CustomerRestController {
    private final CustomerService customerService;

    @GetMapping("/{id}")
    public Customer getCustomer(@PathVariable Long id) {
        Optional<Customer> customer = customerService.getCustomer(id);
        if(customer.isEmpty()) {
            return null;
        }
        return customerService.getCustomer(id).get();
    }
}

Controller Response with Model object

@Slf4j
@Controller
@RequiredArgsConstructor
@RequestMapping("/customer")
public class CustomerController {
    private final CustomerService customerService;

    @GetMapping("/{id}")
    public String getCustomer(Model model, @PathVariable("id") Long id) {
        Optional<Customer> customer = customerService.getCustomer(id);
        if(customer.isEmpty())
            log.error("Customer with id {} not found", id);

        model.addAttribute("customer", customer.get());
        return "customers/customer-profile";
    }
}

I hope to explain myself and my question. Sorry about my english. Thank you :)