Copying "Sketches" saves time since in complex model building, sketch creation and constraining takes significant time. There are numerous times when you need to copy a sketch you already completed in prior model to a completely "new model". New model could be in different work space. You could copy and paste to a "face" of new models as well.

See following related videos.
FreeCad Tutorial. How to reuse a Sketch in Freecad. Carbon copy or cut and paste sketches.
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Timestamps

00:00 Introduction
02:14 Copy Sketch
04:24 Bring to New Model
08:02 Copy Sketch to Face of New Model