No rhyme or reason as to when they send them, or who receives them. Most of the time they are valid in the "off-season" as opposed to peak times.
We've gotten them before and used them for free dining and discounted rooms. Are you on their mailings lists?
The first time I got one I went to their website and built a trip, but did not buy it. Ordered the planning DVD and all that. A couple months later they sent me a pin code valid for 3 weeks around the time we had built the trip that I didn't buy. Anyway, like I said no body knows how they decide who gets them, but it doesn't hurt to do things like that.