(I know, I should be up to 9 or even 10 by now)
Dungeons and Dragons has alignments. Vampire the Masquerade has paths of morality. Judeao-Christians have the ten commandments.
There are paths that govern our actions in the real world and there are paths that govern our character's actions in the imagined worlds of our roleplaying sessions. Different games handle Morality in different ways, sometime regimenting the concepts into the rules and experience systems, and sometimes leaving the area of morality as a "fruitful void" to be explored through the emerging situations as story, mechanisms and players intersect.
Why do we need morality systems in a game?
The simple answer is that we don't. There have been plenty of D&D campaigns that have done away with the concept of alignment, ands the concepts of "humanity" and "morality" have been hand-waved in a vast number of Vampire campaigns that I'm aware of. Some games don't even bother with the pretence of an alignment or morality system.
A more complex answer involves getting into the head-space of our characters. I know a certain player who has commonly stated that when he plays a game, he doesn't play his character interacting with other characters, he just cuts out the middle and deals directly with the other players. He is of the strong opinions that a single player will always play the same types of characters, whether they seem to be playing a "warrior", a "mage", a "nosferatu vampire", a cyberpunk "nomad" or a high fantasy "aristocrat". A leopard doesn't change it's spots.
A morality system helps us break the mould of our own thought patterns. It forces us to consider the choices inherent in a situation from a new perspective. If a character gains some kind of mechanical bonus from acting according to a certain set of principles, then we start to take those principles into account with every action and thought.
A hard-core fundamentalist Judaeo-Christian might feel pains of guilt and distraction from performing deeds against the rulings of the Ten Commandments (and a game system might reflect this by penalising future actions until atonement is made or forgiveness sought). A passing citizen with an awareness of the faith might not feel the same inner turmoil, but if they understand that the local community follows this morality then their actions should suit the rules expressed (or else they suffer appropriate consequences). In the same way an "chaotic" character risks imprisonment when they exist within a "lawful" society, as they have to temper their intended activities...and if they temper those activities long enough, they may start to sway away from their chaotic inclinations. A "good" character in an "evil" society may find themselves trampled by the selfish and power hungry; if they want to survive, they might have to sacrifice their helpful ways and become a bit more ego driven and self-protective.
Artists make choices when they work. A painter may choose a canvas, a type of paint and a pallet of colours. A sculptor may choose a specific stone to carve. They might both aim to embody to same concepts in their work but they need to develop ways of conveying such a thought with their chosen medium. Telling a story through a game is much the same; a game about moral concepts needs a system of morality, or at least a set of in-world laws for the characters to follow. A good morality system does not restrict the actions that can be taken by the characters but it provides ramifications for actions taken outside the chosen morals. Or, it rewards characters who stick to their chosen path.
The keys in the Solar System could work really well as a morality system, where characters might be rewards with taking certain actions in certain circumstances. A canny GM might even make it unfavourable to take these actions in other ways...perhaps taking the moral high ground gives an advantage to opponents following other moral ideals, or gives the edge to those who are willing to fight dirty. Is it worth getting the extra experience point if you might suffer an injury or even die?
Taken simply, D&D morality is a pair of spectra between "Good/Evil" and "Law/Chaos". They polarise the world. In this type of system, two groups designated as "good" will work together against two groups designated as "evil". Good and evil always oppose. But reality is rarely that simple.
This is one of the reasons why I'm trying to veer away from the concept of alignments in my game designs, tending more toward paths of thought that might conflict as easily as they commune.
Morality paths can also be tricky when players choose a certain path to gain specific advantages within a system...using those advantages when they are helpful, but ignoring the path when it either proves too difficult or could cause them a disadvantage. Once morality plays a part in a game, it must always play a part in the game, not just when it suits the players concerned. This is an easy trap to fall into and I've seen it many times.
Morality systems in a game are a dangerous beast. They can bring some great characterisation to the table, but they can also cause some headaches when used incorrectly. Use with caution.