What do the "corner style" settings under "Line" in the "Properties" toolbar do?

I made some lines, using the toolbar at the bottom of the page, then tried out some of the settings under Properties > Line

I observed the cap style setting changing the ends of the line, but the corner style settings don’t seem to do anything.
So I thought “well lines don’t really have a “corner”, perhaps these settings are for arrow heads?” Nope, no change in arrow heads. Square shapes? Nope; changing these settings doesn’t elicit any change in square shapes.

Perhaps these settings are for something else?

You can always draw polygons:

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