C Sharp new learner on View model issue

public async Task<IActionResult> Login(LoginViewModel model, string returnUrl = null)
{
    if (ModelState.IsValid)
    {
      // work with the model
    }
    // At this point, something failed, redisplay form
    return View(model);
}

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Expert please explain me why

One has to validate i.e below

if (ModelState.IsValid)

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Can this view object can be used in services structure where business logic resides.