From a nearer point, to catch Iranian missiles in the boost phase.
Or North Korean or whoever the enemy du jur may happen to be.
A lot of money has been spent to inhibit an Iranian second strike missile response. That's what the radar nuclear shield is about, not preventing Iranian first strikes. They aren't nuts. That shield is to protect from Iranian retaliation.
As could the X51 if stationed close. Easy to detect, hard to disguise, the boost phase of a missiles life is it's most vulnerable.