Order-only prerequisites for emacs documentation

Heime <[email protected]>
Newsgroups gmane.emacs.help
Message-ID <qxsg3oeFzOcGapxsN1Dq0YZIFBT1s0HaCan-gb_QAtViEWBtunl6FRb5fYN2kiamjaQ1cRA5fr52ZehyzgL1FenOgk47JcepSMeQQav1ICU=@protonmail.com>

I notice that Makefile.in includes ${buildinfodir} as order-only prerequisites.
Why is this, why is it important?  Should one do the same for other documentation?

$(buildinfodir)/emacs.info: ${EMACSSOURCES} | ${buildinfodir}
	$(AM_V_GEN)$(MAKEINFO) $(MAKEINFO_OPTS) $(INFO_OPTS) -o $@ $<

But for emacs.dvi and emacs.pdf, ${buildinfodir} is not used.  What is the reason?
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