Bug #63777 documented this for most cases real users will care about, but Branden brought up a situation Obfuscated Coders may need to know about (see comment 3 over there).  The rest of this is his words, lightly redacted to remove an Heirloom troff oddity that has no bearing here:

A horizontal motion of zero length ... is output, and I would expect it to cause a page transition if one is in the twilight zone.  And it does.

$ cat ATTIC/h.roff
.tm 1: nl=\n(nlu
\h'0'
.tm 2: nl=\n(nlu
.pl \n(nlu
$ nroff ATTIC/h.roff >/dev/null # groff 1.22.4
1: nl=-1u
2: nl=0u

I think the manual, as it gets on toward the end of the formatter reference (currently, chapter 5), needs to start discussing the distinction between formatter instructions that change the environment, those that produce output, and those that do neither (register, string, macro definitions and manipulation).

Eventually this background will better prepare the advanced groff reader to grapple with the internal "node" concept.