You are either confused or you know very little about this stuff Hortlund. I suspect it is a mix of both.
Somehow i feel i need to explain it so even you can understand.
It is a deterrent because:
1) It makes your potential enemy aware that if he should fire a ballistic missile against you it will not reach its intended target and you will be able to retaliate.
2) The shield would make your enemy manufacture alot of fairly costly missiles to make sure that enough missiles gets through to get the job done. That in itself should make the enemy think twice before investing.
It is a reason for the shield to be a hot topic when it comes to nuklear arms reductions, and the reason is that...
...it is in fact a deterrent
was it simple enough for you Hort?