Well, see several long paragraphs in MS word, for a start, what with all the space stolen by border decoration, status lines, ribbons, and icon trays.
Or work on a decent size spreadsheet without feeling I'm looking at it through a letter box.
In 2004 I had a dell portable a 14" screen and 20xx by 16xx pixels, and it was dead easy to display two Allen Braldey PLC loader-monitors side by side, and display lots of rungs as well as the debug status window below them.
in 2011 I have a jujitsu 1600x900 and I can only display one rung at a time, even though I have dragged windows icon bar and start menu to one side. I can't display the analogue function blocks at all in any worthwhile way.
Then there is displaying enough of an A3 size block diagram at once to be able to see how the bits go together.
Try designing a SCADA screen mimic with less pixels for the customer screen and the various design toolbars than the final customer screen will have by itself. The slow, tedious, repetitive scrolling will drive you to despair.