The sketch is the basis for 3D modeling and design. When you open a new part document, you first create a sketch.
Sketching has other critical uses in large scale designing.
You can, for instance, use blocks to create layout sketches. Layout sketches are useful for many purposes, including creating the layout of complex machinery, or to outline system representations.
3D sketches are used to generate sketch entities in space for creating weldments and piping systems for example.