Перед проверкой необходимо сначала проверить маршрут ....
$roles =[
'title' => 'required|unique:posts|max:255',
'author.name' => 'required',
'author.description' => 'required',
];
if(Route::getCurrentRoute()->getPath() == "xxxxx"){
$role['desc'] = 'required'
}
if(\Request::route()->getName() == "yyyy"){
$role['desc'] = 'required'
}
if($request->is('admin/*')){
$role['desc'] = 'required'
}
$this->validate($request, $role);