미니프로젝트 질문입니다

package kr.co.softcampus.controller;

import javax.validation.Valid;

import org.springframework.stereotype.Controller;

import org.springframework.validation.BindingResult;

import org.springframework.web.bind.WebDataBinder;

import org.springframework.web.bind.annotation.GetMapping;

import org.springframework.web.bind.annotation.InitBinder;

import org.springframework.web.bind.annotation.ModelAttribute;

import org.springframework.web.bind.annotation.PostMapping;

import org.springframework.web.bind.annotation.RequestMapping;

import kr.co.softcampus.beans.UserBean;

import kr.co.softcampus.validator.UserValidator;

@Controller

@RequestMapping("/user")

public class UserController {

@GetMapping("/login")

public String login() {

return "user/login";

}

@GetMapping("/join")

public String join(@ModelAttribute("joinUserBean") UserBean joinUserBean) {

return "user/join";

}

@PostMapping("/join_pro")

public String join_pro(@Valid @ModelAttribute("joinUserBean") UserBean joinUserBean,BindingResult result) {

if(result.hasErrors()) {

return "user/join";

}

return "user/join_success";

}

@GetMapping("/modify")

public String modify() {

return "user/modify";

}

@GetMapping("logout")

public String logout() {

return "user/logout";

}

@InitBinder

public void initBinder(WebDataBinder binder) {

UserValidator validator1=new UserValidator();

binder.addValidators(validator1);

}

}

진하게 된 @ModelAttribute질문입니다

@ModelAttribute는 html로 넘어오는 값을 자동으로 클래스에 담아주는 역할을 한다고 들었는데

join_pro메서드에서 다시 또 한번 써주는 이유는 무엇인가요?