Sweep would also work - you can almost think of extrude as being a special kind of sweep along a line.

But for sweep you have to draw an extra line - extrude does the job with fewer steps because you don't have to draw the line for the path like you would in sweep.

Often times there are many different possible ways to create a particular shape. Like another totally different way for your half cylinder would be to use Draw solid / Cylinder to create a full cylinder, then draw a line in the front or side view and use Construct / Boolean / Difference to slice the cylinder in half with the line and throw away the bottom half...

But some commands are optimized for making certain shapes with fewer steps, like extrude is efficient for doing things that have a straight line path in them.

- Michael