Lines are other elements of the layout. They can be used to:
Highlight or underline something, as for titles
Join or link elements, for example, when set with arrows or other kinds of ending
Give a custom graphical design
Be used as guides to place objects, especially because Scribus can't have built-in rotated guides, so lines can be used instead
Some years ago it was very common to see documents using lines as the most important graphical element of the layout. It has now virtually disappeared, maybe because those kinds of elements are hard to use, and it really calls for some different kind of creative approach. But they are still used in many other cases.
You can count on three Scribus tools to help you create lines and a window to customize them as you wish. They all have common settings, share line styles, and can be defined in the Line tab of the PP and the stroke color can be applied to them, whereas fill color...