Comment # 1 on bug 1222446 from Johannes Meixner

See
https://build.opensuse.org/projects/Printing/packages/cups-filters/files/cups-filters.changes?expand=1
that reads (excerpt):
------------------------------------------------------------
Thu Mar 13 11:36:37 CET 2014 - jsmeix@suse.de
- Added "Recommends: poppler-tools" because /usr/bin/pdftops
  (provided by poppler-tools) is needed (but not required
  for non-PostScript printers) to print PDFs on PostScript
  printers (bnc#868148).
------------------------------------------------------------
where 'bnc#868148' is
https://bugzilla.suse.com/show_bug.cgi?id=868148
which is a non public SLES12 bug
where its initial comment reads (excerpt)
------------------------------------------------------------
cups-filters RPM spec file shoud contain
  Recommends: poppler-tools
because /usr/bin/pdftops (provided by poppler-tools)
is needed to print PDFs on PostScript printers
because in this case the CUPS filter chain is:
  /usr/lib/cups/filter/pdftopdf
  /usr/lib/cups/filter/pdftops
  where /usr/lib/cups/filter/pdftops calls /usr/bin/pdftops
  /usr/lib/cups/backend/...
------------------------------------------------------------
which is also shown as a comment in
https://build.opensuse.org/projects/Printing/packages/cups-filters/files/cups-filters.spec?expand=1
------------------------------------------------------------
# /usr/bin/pdftops (provided by poppler-tools)
# is needed (but not required for non-PostScript printers)
# to print PDFs on PostScript printers because in this case
# the CUPS filter chain is:
#  /usr/lib/cups/filter/pdftopdf
#  /usr/lib/cups/filter/pdftops
#  where /usr/lib/cups/filter/pdftops calls /usr/bin/pdftops
#  /usr/lib/cups/backend/...
# see https://bugzilla.novell.com/show_bug.cgi?id=868148
Recommends:     poppler-tools
------------------------------------------------------------

Regarding why "Recommends: poppler-tools"
instead of "Requires: poppler-tools" see
https://bugzilla.opensuse.org/show_bug.cgi?id=776080#c39
(which is an issue about CUPS)
which reads (excerpt)
------------------------------------------------------------
RPM Requires are hard dependencies
that cannot be skipped by end-users
(without having unresolved dependencies in their systems)
and cups is installed on almost any system
(except minimal systems)
so that all RPM Requires in cups result
that all those hard dependencies
must be installed on practically any system.

If I remember correctly the openSUSE build service
installs all RPM Requires to set up the build system
of a package so that all RPM Requires in cups result
that all those hard dependencies must be installed
in our build systems for any package that directly
or indirectly BuildRequires cups.

In short:
Keep RPM Requires as small as possible - i.e. only what
is really mandatory to let the software run - and specify
all what is optional (i.e. what is not really mandatory)
as RPM Recommends.
------------------------------------------------------------

For cups-filters those text would be
------------------------------------------------------------
RPM Requires are hard dependencies
that cannot be skipped by end-users
(without having unresolved dependencies in their systems)
and cups-filters is installed on most end-user systems
(i.e. all end-user systems with printing functionality)
so that all RPM Requires in cups-filters result
that all those hard dependencies must be installed
on almost all end-user systems.
------------------------------------------------------------

When you do not have the poppler-tools package installed,
you do deliberately refuse recommended packages
so you have to specify on your own what
you need in your particular case.

You cannot have both at the same time:
An automatism that also installs
what is recommeneded for usual functionality
and at the same time
refuse what is recommeneded for usual functionality.