When you stretch you should achieve and maintain a neutral position in the joints and spine in order to isolate the tissues you're targeting. The half-kneeling position should make you achieve centrated and neutralized joint positions at the shoulders, hips, and spine.
It should also transfer these positions into useful strategies to improve the response of your rectus femoris and deep ilio-psoas group, not only a mechanical standpoint but also to "release" neurological based tone and tightness.