Header files are bits of PostScript included in the output file; generally they provide support for special features, rather than producing any printed output themselves. You can explicitly request downloading header files if necessary for some figure, or to achieve some special effect.
Dvips includes some headers on its own initiative, to implement features
such as PostScript font reencoding, bitmap font downloading, handling of
\special's, and so on. These standard headers are the
`.pro' files (for "prologue") in the installation directory
`$(psheaderdir)'; they are created from the `.lpro' ("long
prologue") files in the distribution by stripping
comments, squeezing blank lines, etc., for maximum efficiency. If you
want to peruse one of the standard header files, read the `.lpro'
version.
The PostScript dictionary stack will be at the `userdict' level when header files are included.