WP Query vs. Pre_Get_Posts

 /* Modify the Query Directly */
$today = date( 'Y-m-d' );

$args = array(
   'post_type' => 'bgmp',
   'meta_query' => array(
     array(
         'key' => 'coupon_end_date',
         'value' => $today,
         'compare' => '>=',
         'type' => 'DATETIME'
     )
 )
 );
$the_query = new WP_Query( $args );

/* OR use pre_get_posts */

add_action( 'pre_get_posts', 'fm_modify_coupon_query' );
function fm_modify_coupon_query( $query ) {

    // Check to make sure we're on the front end, is main query, and within the coupon taxonomy
    if( ! is_admin() && $query->is_main_query() && is_tax('coupon') ) {

    $today = date( 'Y-m-d' );
        $query->set('meta_key', 'coupon_end_date');
        $query->set('meta_value', $today);
        $query->set('compare', '>=');
        $query->set('type', 'DATETIME');
    }

}