What is the modeling technique where a line is used for extrusion?

I’m currently learning 3D modeling in Blender and noticed something interesting in some tutorial videos. In these tutorials, the creators start by drawing a line, which they then extrude to help shape their models quickly.

I’d like to understand what this technique is called and how it works, as most other tutorials I’ve watched focus on starting with basic shapes like rectangles and cylinders for modeling heads or similar structures.

Could someone clarify what this approach is and when it’s typically used?