It seems the Rosetta spacecraft was put (put itself) into hibernation 08-June-2011. It spun itself up to stabilize its attitude, then shut down navigation, attitude control, communication systems as well as some of the heaters - if I understand correctly.
It then remained in hibernation for about 2.5 years (by far the longest scheduled hibernation/wake-up on record!), with only the (a) computer heated and powered, but probably without a significant workload. This was powered by the small amount of light that fell
intermittently on the panels at various angles which were pointed towards the sun. The rotation allowed for passive maintenance of spacecraft attitude.
It then woke itself up 2.5 years later on 20-January-2014, de-spun, re-oriented for better sunlight on the panels, and "phoned home."
The video below is great, and this answer is helpful to read - including rosetta_wake-up_faq_20_jan_2014.pdf about Rosetta coming out of hibernation. However, it seems to explain that the "special hibernation mode" was necessary to survive the hibernation itself, but I don't see why that in turn was necessary.
A special ‘hibernation mode’ for the spacecraft was designed by engineers at EADS Astrium, the main industrial prime contractor that built Rosetta, to allow it to survive the large distances from the Sun during the cruise.
Was the spin done to conserve propellants? Lifetime of the reaction wheels? I can't think of anything else that would get "used up" by leaving it active.
WHY did Rosetta have to go into hibernation for 2.5 years?
I measured some screen shots of this video - Rosetta is only about 18% farther from the sun at aphelion (middle of hibernation) than it is when it enters and exits hibernation. That means sunlight at perihelion was about 72% as bright as at hibernation enter/exit.