When users write text documents, the paragraph's style that they need to use is
"text body". Why not "Default"? Because "Default" is the root style and that
every style inherit from it.

Here is a typical use case:

a user does not know styles when he starts writing the document. He does not now
title styles, so he sets the title using "hard" formatting (setect the text and
sets fonts, bold... mannually).

Then he discovers styles. He firtly sets the titles with styles. Then he wants
to change the background color of all the text paragraphs but not the titles.

As the text paragraphs are by default "Default", if he changes the bg color,
even the bg color of the titles will change which is annoying ("Default" is the
root, and thus the parent of all the titles). If the default style is "Text
body", everything would work like a charm.