Adding to the existing answers: The triangular flight paths were needed to get enough images of the comet's surface, which are required for the low paths Rosetta flies today (and for dropping Philae close to the surface).
Rosetta's orbit propagator is an innovation insofar as it uses landmarks on the comet (among other things) to find the comet's and its own orientation, and to find a route to a given target landmark location. For this, the comet first had to be imaged in great detail, and that happened during the "triangle phase."
This has never been done before, and makes Rosetta a very exciting pilot project for ESA's future far-reaching space missions, which will all use this orbit propagator. It is already a breathtaking success if you consider the complexity the software has to manage.
Also, except for planned crash "landings," no spacecraft has ever flown by (or, as Rosetta does all the time now, has flown around and around) a comet in altitudes as low as 10 km. This requires a completely new auto-navigation system, and that one needs a map of the comet that cannot be acquired from earth (or from great distances in general).
(It is called "orbit propagator" despite the fact that Rosetta is not in an orbit because it is built upon other components that were core parts of "real" orbit propagators.)
http://issfd.org/ISSFD_2012/ISSFD23_IN2_2.pdf sheds some light on the propagator. Recommended reading imho.