Why use mobx gist9

gist.js

class Form {
 @observable
 email = ""
 @observable
 password = ""
 
 @computed
 get emailError(){
   if (this.email && this.email.length >0 &&
       this.email.indexOf("@") >= 0){
       return "email must contain @ character"
   }
   return null
 }
 @computed
 get passwordError(){
   if (this.password && this.password.length >0 &&
      this.password.length < 8){
      return "password must have 8 characters"
   }
   return null 
 }
 @computed
 get valid(){
   return this.email && !this.emailError &&
     this.password && !this.passwordError
 }
 
 @action
 setEmail(event){
   this.email = event.target.value;
 }
 
 @action
 setPassword(event){
   this.password=event.target.value;
 }

}

return function FormView(props){
 const {submitForm} = props;
 const form = useMemo(()=>new Form(),[true]);
 return <form onSubmit={()=>form.valid && submitForm(form)}>
   <input name="email" value={form.email} 
     onChange={form.setEmail} />
   {form.emailError && <div>{form.emailError}</div>}
   <input name="password" value={form.password} 
     onChange={form.setPassword} type="password"/>
   {form.passwordError && <div>{form.passwordError}</div>}
 </form>
}