hbox_set:Nn ⟨hbox⟩ {⟨contents⟩} typesets but doesn't print the ⟨contents⟩

In the interface3.pdf documentation, it is said:

hbox:n {⟨contents⟩} Typesets the ⟨contents⟩ into a horizontal box of
natural width and then includes this box in the current list for
typesetting.

hbox_set:Nn ⟨hbox⟩ {⟨contents⟩} Typesets the ⟨contents⟩ at natural
width and then stores the result inside the ⟨hbox⟩.

I wonder what is the meaning here of "Typesets the ⟨contents⟩", since the latter doesn’t print anything, unlike the former which, admittedly, is said to "include th[e] box in the current list for typesetting".

documentclass{article}
begin{document}
ExplSyntaxOn
hbox:n { Foo }
hbox_set:Nn l_tmpa_box { Bar }
ExplSyntaxOff
end{document}