For example, let's say that you think the taxes that you're paying are distributed unfairly. A few landowners get together, and then get the poor citizenry to go kill some redcoats, and voila, you've got a country where the rich people get to set the taxes.
Or, perhaps you need more resources, and there's an entire continent out there waiting for your development. You get the poor people together, call them settlers or get them into the army, and send 'em out to kill injuns, or later, you run a war against Mexico. You get the resources, and the poor people get killed.
But dammit, that's not enough. Spain's cruising around with all these Islands, and they could be profitable too- well, once again, send the poor and middle class out to kill some Spanish. Score some commonwealths, protectorates and territories. Sweet!
I could go on and on. For example, you need an excuse to get a war on, so that a flagging economy gets a boost and inflates. So you deliberately ignore intelligence warning you of an attack, and 3,000 citizens die, so that you can send more citizens out to die.
But it's okay. You're going to get oil. Lots of it too. That's cool, right? No other way to get it, except sending other people to die.