The original LaTeX provided the
\include command to address the
problem of long documents: with the relatively slow computers of the
time, the companion
\includeonly facility was a boon. With the
vast increase in computer speed,
\includeonly is less valuable
(though it still has its place in some very large projects).
Nevertheless, the facility is retained in current LaTeX, and causes
some confusion to those who misunderstand it.
In order for
\includeonly to work,
\include makes a separate
aux file for each included file, and makes a “checkpoint” of
important parameters (such as page, figure, table and footnote
numbers). As a direct result, it must clear the current page
both before and after the
\include command. (The requirement
derives from the difficulties of
observing page numbers.)
What's more, this mechanism doesn't work if a
appears in a file that was
\included itself: LaTeX diagnoses
this as an error.
So, we can now answer the two commonest questions about
Answer: because it has to. If you don't like it, replace the
\include command with
\input - you won't be able to use
\includeonly any more, but you probably don't need it anyway, so
\included files? - I always used to be able to under LaTeX 2.09.
Answer: in fact, you couldn't, even under LaTeX 2.09, but the failure
wasn't diagnosed. However, since you were happy with the behaviour
under LaTeX 2.09, replace the
\include commands with
\clearpage as appropriate).