I assume it is a technique to Get you to psychologically think about attending there…if you feel it is something scarce (get it now before it is gone!) then you tend to put more weight on it. Also if you have put a housing deposit down then you have anchored your thoughts to attending there.
Also they may make money this way.
If there is not enough housing or there is less desirable housing, then this will allow you to claim housing.
But I think this a dubious practice!