It's the technology/sociology pacing that I think they are concerned about maintaining more than anything else. The idea is that a civilization left to its own devices will either see social development that matches or outpaces its technological development so that it uses those technological developments responsibly (enough) or it will destroy itself. If an alien civilization interferes and introduces more advanced tech than they are socially ready for, there is a strong chance that they will use it irresponsibly either to wipe themselves out, or, to become galactic a-holes.
The idea that the Klingons are actually a medieval-level civilization that had Starfleet-level tech introduced to them by another civilization that enslaved them (which is itself a socially misguided use of the tech, and probably an indicator that, much as child abusers were often abused, themselves, when they were children, this sort of problem propagates itself - the Hurq or whatever they were called were probably also introduced to the tech too early by some other spacefaring race) probably played strongly into the decision to create and enforce the PD.
People think the PD is harsh, and in some cases it is, but I can see a way it could be worse: what if the Federation monitored cultures that weren't ready for contact, analyzed their social development, and sent black ops teams to them to eliminate technologies (and tracelessly abduct/eliminate their developers in "accidents") that they determined that the world wasn't socially ready for? Imagine their people being 'disappeared' whenever they tried to develop powder weapons, or nuclear weapons, or fast food distribution!