A few thoughts:
When I got into the hobby in the 70's I wanted a yard, shortly after I built my first layout(with my Dad's support). Why? To show off my collection of equipment, + because a 'yard,' was a prototypical 'layout design element,' at that time. Bruce Chubb's first book on Operation came out about that time. Shortly after that was the RMC V&O article series, which focused my attention on more 'prototypical' aspects of railroad operations.
With later layouts, I had basically storage, or staging yards, to feed trains onto the layout, + a small classification yard, to swap blocks, and do light classificaiton.
Today, I still plan on including a staging yard, but also looking at how the yard will be used, in terms of classifying freight for local industries, and blocking outgoing trains for beyond the basement destinations. Today, I'm concerned about having the right number of tracks, to do all this, plus keep the yard fluid. Won't know for a while whether I've succeeded or not.
In terms of whether I think every layout needs a yard, my answer is it depends. If the owner is interested in mainline traffic, then they'll probably need a decently designed staging yard, that directs trains into and off of the proper track. As for classification or sorting of cars for local delivery or pick up, depending on a layout, it might just be an extra siding in a team track, or station area.
As for having a properly designed yard, and a properly designed yard for a given layout, they aren't the same thing to me. May be semantics to some, but a properly designed yard has sufficient infrastructure to support fluid operations. A model railroader if he or she doesn't have sufficent space, but wants or needs to include a certain location on his/her layout, may elimiate portions of a prototype yard, due to space or scenic considerations, say if scenery is more important to him/her.