Frank Ellison, one of the great pioneers of realistic operations on a layout, in a series of articles in Model Railroader magazines starting in the late 1940's, (IIRC), or the early 50's felt that the layout itself was like a stage, with the trains as actors. They had actions to perform while they were "on stage", either switching industries, entering the yard at Fillmore on his layout, where the trains were broke back down and remade, the power was serviced etc, entering from stage right, or stage left, and exiting off stage to either side after they had "performed". He called this area where the trains got ready to go as staging.
My personal layout is designed on these principles Frank Ellison developed. So I tend to believe that some kind of yard, staging or otherwise is needed. A one track, visible, yard can be a requirement on a small layout as I said earlier, but to help simulate more varied traffic, a staging area or as the British call it a "fiddle" yard, is needed as well. This can be just a siding on the back of the layout, or even just a straight part of the main back there. But it provides a place to rearrange or to break down trains that provides a more realistic variety of traffic on the main "stage".