While alignment systems aren't necessary, they're good for helping you ground a character in a game. I find that they're most useful in a first playthrough, especially if you're not familiar with the setting. In a first playthrough, I at least rarely have as deep an idea for my character as I do in later playthroughs. So alignment choices give me something to fall back on, a sort of "when in doubt, pick this" option I can use when I'm not sure what my character would do in a situation. I find that they're also a nice way for a game to telegraph that a thing matters sometimes. They can help you understand what the game will react to.
