I've followed racing (cars and bikes) for years, and taken it for granted that the fastest qualifier would take pole position and start at the front of the grid. A friend asked me why that is, because he thought it gave an additional unfair advantage to drivers that are already proven faster. I realized I could speculate as to why (e.g. less frivolous passing makes the race safer, faster drivers are not bogged down and forced to lose by being made to pass many times in a limited-time race, simply a merit-based system, etc) but didn't have a concrete answer.
Friend asked why the fastest riders aren't placed at the back of the grid to force them to prove their merit. Easy answer is, everyone would sandbag the crap out of qualifying, no one would give their best because they'd want to be closer to the front. Friend asked why the grid isn't just randomized. My answer is that it removes all merit from the process, as well as the other safety-related item mentioned above. But I realized that's all my own speculation as to the "why" question.
So is it "just the way it is," is it tradition, just a decision that was sort of arbitrarily made, or is there another reason why the fastest driver gets to be on the pole?
I'm aware that there are systems where the fastest qualifier isn't just given the pole, but for the purposes of this question, I'm asking what are the reasons (if any are known) that many high profile racing leagues/series place the fastest driver on pole?