They include a collection of Type 1 fonts for printing and display.
X provides no means to get real font metrics information,
so we can't print using X fonts. X can read Type 1 fonts, and
we can embed them in PostScript output, so we ship them.
TrueType fonts are supported by some X servers and X font
servers, but we don't have a good way to get them into
PostScript documents now.
-- Shaw Terwilliger