why we declare indexfields in model in laravel code example

Example 1: laravel migration remove column

public function up()
{
  Schema::table('table', function($table) {
    $table->dropColumn('column_name');
  });
}

Example 2: schema add column laravel

Schema::table('users', function($table) {
    $table->string("title");
    $table->text("description");
    $table->timestamps();
});