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Depending on the RPG you play some may require you to align yourself with some kind of system to broadly (or narrowly) define your character's outlook on morality. This goes back to the the 3x3 Law/Chaos/Good/Evil Alignment matrix and filters up through to more modern approaches that include motivation systems, morality paths, and even simple linear alignment spectrums. Other games have systems that balance power and a character's inherent humanity, or balance two sides of a character like the internal beast and the civilized human. Still others balance sanity and madness. All have in common some degree of intent behind the way the character acts.
Is there any value in these behavior systems? I think it depends heavily on the game, and obviously some games have jettisoned these concepts entirely, and and none the worse for it.
Some of the dungeon crawler games hold onto alignment, either the aforementioned classic 3x3 array, or a simpler Law-Neutral-Chaos linear scale. While these games often have some mechanics that interact with alignment, including magic weapons, clerical or divine abilities, and similar, outside of these effects alignment is little more than a vestigial trait with only the most tenuous of ties to game play.
Conversely are games where morality is tied heavily to the game intent. Fantasy Flight's Force and Destiny tracks morality as a "good vs evil" or "light vs dark" balance. Actions that tap the dark side slide the character further toward one end of the spectrum, and holding to the virtues of the light side elevate the character toward being a paragon of "good." Force sensitive characters must find their place between the light and dark sides of the Force whether that this fully embracing one side to the exclusion of the other, or trying to balance the two in a "grey Force" technique.
Meanwhile, White Wolf and Onyx Path have made it their signature to have games that balance characters along a path of morality or one where humanity lies on one end of the spectrum and the character's monstrous/supernatural self lies at the opposite end. This monstrous nature is often a balance between the instincts of the monster/supernatural self, and their own core beliefs. A character closer to the vampire end of their moral spectrum ceases to see the drinking of blood (and possibly the taking of life to feed their thirst) as "evil" and instead understands it as a case of their nature. The vampire can struggle to retain their humanity (and often hamper themselves in some way) or can embrace some degree of their supernatural nature as "natural."
But do morality & behaviour systems improve our games? Like a lot of things I think there's a place for these in some instances and not in others. Fantasy Flight wisely chose to not have Light and Dark side morality in their Edge of the Empire and Age of Rebellion games. Since Force users in those games were only a minor aspect of the whole, a mechanic like morality would have been out of place. For Force & Destiny however it is an integral piece of stories that focus around Force users, and the mechanics add to that part of the game rather than detract.
Likewise, where one game of playing a once-human monster like a werewolf or human may revel in the power of the supernatural, another may instead choose to dwell in tales of lost humanity caused by the corruption by the supernatural. In the former case a behavior system (be it morality, humanity, or something else like "balance") would likely get in the way of having that power trip of playing a monstrous character. In the latter where the temptation of supernatural power balances against the loss of humanity from the corruption that power brings may feel lacking without a behavior system.
What about Madness and Sanity? These balance against each other but unlike many of the examples above madness is often foisted upon a character; an imposed condition caused by outside forces. Instead of an internal struggle determined by the choices of your character madness and sanity are an external struggle of the self versus a universe that threatens to drive the self mad. Games set within the Cthulhu Mythos would hardly be the same without rules for madness and insanity.
All of these systems will drive players to play their characters in certain ways, ways that may well change over time. Some at the choice of the player, and others as a result of forces beyond their control. In the end a behavior system needs to fit the game, and fit the players and GM. Like many things agreement by all involved and a clear understanding of what goals the GM has in mind for the game can help to ensure that these kinds of things fit within the game and fit within the expectations of all involved.