I like john's givens and druthers. I do disagree that yards especially and engine terminals take up space and are used as parking places. if you are interested in operations, a yard can be a big part of it, and definitely not a storage lot. For example, on my layout I have a 5 track yard that can hold about 60-70 cars. My operations include through freights that come from some staging and continue to staging. They drop off and pickup cars at the yard. These are cars going to or from industries modeled on the layout. Then local freights originate at the yard, getting engines from the engine terminal and deliver and pickup cars from the local industries. In a normal operating session of about 3 hours, every car in the yard is changed.
So yards and engine houses may certainly have a use on an operating layout.
Another very important consideration in my opinion in some form of connection to the rest of the world. This can be a staging track, hidden or not, that holds a train. Or it can be a track designated as an interchange with another RR that just holds a string of cars. Having an outside connection gives your operation a purpose and provides a place to send any kind of car. And if you have two of them, you can send car types not used on the layout, from one to the other justifiably.
