It could be the realtor is trying to make you use them as they could get you to think that only they can tell you where it is located.
The answers below are likely correct. And some agents get lazy when they put out listings. Maybe they just forgot to put an address in the description.
I have also read that on occasion the sellers do not want to be bothered. They actually insist the location is not disclosed. That way they do not have people coming around all the time.
I don't agree with the whole concept of not putting down an address, if its someones exclusive they have no reason not to display it. When it it's an Open Listing, let's say a FSBO, these brokers can't co-broke, so they sit and wait for potential buyers to call them.
What were you looking at that caught your interest?