Freight cars sitting in a classification yard aren't making money, and tracks full of cars not yet classified imply delayed freight, disgruntled consignees, and trains held out of the yard waiting their turn.
Here's the Marquette (Iowa) West Yard on the Fox Valley O Scalers Milwaukee Road layout. Patrols have been dispatched to Waukon, Edmore, the docks at South Marquette, and through freights are on their way to Calmar (westward) and Dubuque (eastward). There's plenty of room for the cars that will come in from the docks and off the returning patrols.
With another subdivision yard now open at Calmar, expedited train movements are now possible, such as this solid livestock train about to make a priority pickup at Ossian.
The Calmar yard is where crews sort cars for solid meat trains, which makes the life of the Marquette yardmaster much easier. Run the icers past the ice dock, pull the shorts and the La Crescents off the hind end, tack any iced cars that have arrived off the patrols on, get 'em out of town. The dispatcher has to stay sharp, though, not letting dead freight with work at several online towns get in the way.