By the way, I just want to note that although yield is the accepted term for college admissions offers being accepted, I think sometimes that connotes to people that the college is targeting some sort of statistical result. I am personally skeptical that is often, if ever, the case. I think really they just want to enroll the best possible classes.
The problem is if you admit someone and they don’t take your offer, now you need someone else to enroll. Even then, if you have reasonably predictable yield and there are lots of that sort of person you could admit, then this is not such a problem as you can just admit enough that after yield you enroll enough.
And my understanding is this is how most colleges mostly work in fact. Even if they have ED (and many don’t), they don’t get enough applicants ED to enroll more than a fraction that way. So, they just admit a lot of kids RD, maybe (or maybe not) use merit to improve their odds, and then enroll a class.
But for some colleges, apparently yield is not so predictable, or not for certain sorts of applicants. Like maybe your yield is pretty predictable for applicants in the lower end of your normal range, but not for applicants toward the top or indeed above your 25ths and such.
In cases like this, if you don’t admit enough people you might have to use your waitlist a lot, and that could mean not getting the best class you could have gotten. If you admit too many people, you are overenrolled and you are putting third beds into doubles and such.
So–now maybe you look to ED to reduce the uncertainty. But you still need to get enough people to apply ED. And again I think a lot of colleges just don’t get the ED applicants they need to do much of this. But maybe you do, or maybe you can with enough promoting and pushing and so on. And you find that way you can do less going to the waitlist without overenrolling.
None of this is great for the kids, but I do think it is usually not so much about the stats. It is about trying to get the kids they really want.