PC "Back story", why is that a thing?

[Ninjaxenomorph]

Why backstory? Because it's one of the best tools for determining how my character's actions would come down, and how they would differ from my own.

Hmm, I'll use a few of my recent PF character ideas. In a recent aborted game, I was playing a Dhampir named Thaddeus. Now, one of the interesting things about dhampir is that they almost demand backstory. Other half-breeds can be run-of-the-mill, but if you're a dhampir, how you came to be is usually a big question. Is your mother still alive? Do you know who your father is? Do you hate vampires, or want to become more like them? Hell, a mundane backstory is even weird if you're a dhampir.

Anyway, our group was in a position to learn of a plot by the local church to assassinate the king. Out of character, plot bells were going off in my head. In character, Thaddeus wasn't local, but he's the son of a lord in a nearby vampire-ruled kingdom. He came to this land to learn how to better fit in, in addition to the standard 'find fame/fortune' reasons someone goes adventuring. He figures that, if he saves the king, he gets leverage, right? Improve relations between the local kingdom and his homeland, gain political clout, and make daddy 'proud'. At least enough to elevate him to full vampire. Amongst the rest of the group, if I were to have to convince them, knowledge of their backstory would have helped. One guy was playing basically a samurai; if he was reluctant, I would say something along the lines of 'but doesn't your honor demand you save this rightful king?'

Personally, I like Thaddeus the foreign noble eager to prove his worth, rather than Thaddeus the warlord that hits things and wants to make money without offending his player.

Let's use another character of mine, cold open this time. Wreave is in a border-town with his party. While perusing the marketplace, they overhear a blind woman getting 'harassed' (more like she was being politely detained) by soldiers. Now, a rational question might be to ask what was going on. Wreave walks up, hand on the hilt of one of his many weapons, and none-too-kindly asks the soldiers to step off. "Give me an excuse."

The reason Wreave was cruising for a fight with professional soldiers was not because I wanted a fight, but because Wreave was, in his backstory, wronged by the country the soldiers in question worked for. Their actions against his family, in his mind, justified his his stance of 'looking for excuses to spill their blood'. I'm not saying developing a characters backstory during play is a bad idea, far from it. But, I'm going to need something to work from.