By and large I don't I don't enforce alignment other my no evil PC policy. Alignment is just one more descriptor amongst many that help define a character.
Where I use it is in determining how NPCs and monsters respond, how they see the world. One theory in psychology is that people view the world through schemas, basically predefined templates for how we view the world. You can read about it here.
What Role Do Schemas Play in the Learning Process? It even extends to color, apparently if you don't have the word for the color blue you don't distinguish between it and green.
There's Evidence Humans Didn't Actually See Blue Until Modern Times
Two people can look at exactly the same situation and form vastly different opinions on what is going on. One person may see a beggar on the street and have empathy for someone down on their luck while another may see someone who is lazy or unwilling to work. Yet another person would see someone that could be abducted and killed and no one would notice.
As another example, a chaotic person may look at how a kingdom is being ruled and see a tyranny where title, not worth determines a person's value. Another may look at the same kingdom and see an organized structure that works for the people by giving them a sense of tradition and a proper place.
There's a lot of leeway in this concept, they are just general guidelines. There may often be conflict - a Chaotic Good character may not like the tight control a king has over his kingdom, but may realize that the option is worse. A Lawful Evil person may work to throw a kingdom into chaos so that in the long term a new regime may rise from the ashes.
In addition, people aren't always 100% consistent. Personal experience may soften the heart of a chaotic evil serial killer for very specific individuals. A lawful good paladin may have a blind spot and not realize the harm they are doing to innocents.
Lawful: views the world as a clockwork mechanism. Everything works according to a grand plan, even if we don't understand that plan. When things are in proper order, the whole system works smoothly. If a title is honorable, the person holding the title should be given the respect the title deserves. In some situations they may decide the person is unworthy of the title.
Chaotic: there is no grand plan. The only organization is that which makes sense for the people involved. If the old order needs to be replaced so that people can be free to pursue their own goals, so be it. Perceived organization comes out of individuals choosing to cooperate for themselves or their community. Individuals should be judged by their worth or power, not by title or station.
Good: this is complex, but essentially it comes down to empathy (the ability to put yourself in someone else's shoes), and not wanting to harm others. This doesn't mean you don't fight or kill, but that you will fight and kill because you need to protect others. You may do things for your own personal gain as long as you are not harming innocents.
Evil: in general evil people view others as objects with no inherent value. They may love someone, but in many cases love them as a possession, something they own. If the object of their affection doesn't reciprocate they may not care. They may kill or cause pain in others simply because they enjoy it. Your personal gain is all that matters, other people's goals do not unless they hold power over you or you can use their goals to manipulate them.
