It provides a possible fallback in situations where
Sys.timezone() fails to determine the system timezone. That can happen when e.g. the file
/etc/localtime somehow is not a link into the corresponding file with zoneinfo data in, say,
Duane McCully provided a nice StackOverflow answer with code that offers fallbacks via
/etc/timezone (on Debian/Ubuntu) or
/etc/sysconfig/clock (on RedHat/CentOS/Fedora, and rumour has it, BSD* systems) or
/etc/TIMEZONE (on Solaris). The gettz micro-package essentially encodes that approach so that we have an optional fallback when
Sys.timezone() comes up empty.
In the previous paragraph, note the stark absense of OS X where there seems nothing to query, and of course Windows. Contributions for either would be welcome.