This is supposed to provide short-term emergency capability before a fully operational missile defense system could be deployed within years. The missiles could intercept missiles from countries such as North Korea.
With N Korea's recent thumb and nose to the US, do you think that this is a coincedence? Although, I would like to see more on how effective one of these things are. Last I heard only one out of four or five tests managed to hit its target. But politicly and as an PR campaign, its a good move against NK. What say you "sure pick on Iraq but North Korea has nukes and you don't do anything against them" crowd. I think we have some saber ratteling left over for them as well.
I'm not saying that we should go to war with NK, and I don't know if this missile will work as touted, but its a good political move both internationally and domestic.