Drag a light color window (empty textpad) with a mouse very rapidly left and right across the dark desktop. You should see the tearing if you don't use a compositor. Tearing looks like the window is made of two unaligned halfs (top half and bottom half). With a compositor, the window is always perceived in one whole piece.

I assume the full window content is visible when dragging, not just a border rectangle as sometimes happens when the compositor is disabled.

Also, not everybody notices it, just like not everybody notices text with bad kerning or individual instruments and percussion elements in a song.