The w3-justify class adjusts word spacing so full lines reach both the left and right edges.
The last line normally stays left-aligned.
Compare the two paragraphs and resize the window to see how the spacing changes.
Learning CSS is easier when you can see the effect of each class. A short example lets you compare the same text with different styles. Resize the window to see how the words move onto new lines. The content stays the same while the spacing between words changes.