What's the purpose of the cube?

Putting prequels and sequels aside, keep in mind that Cube is a metaphor for life, for the human condition:

• In the beginning, each character wakes up in a strange, uncomfortable place.

• Each quickly realizes that making a bad move will result in death

• Each reacts to the situation in a unique way

• Those who live longest never get any real insight into who/what put them there, or why

So while the characters (and us viewers) focus on trying to figure out the purpose of the cube, no answer is ever given -- just like in life.

An additional point is that the sole escapee is the mentally disabled man, who just accepted what was going on, and never asked "why" or "who" or "how" -- suggesting the movie's message might be "you're better off embracing the mystery than trying to solve it."

Cube made no extensive attempt to explain what the cube that the characters are confined in is, why it was created, or how the people were selected to be put inside there. The movie remains without any answers.

But Sequels have done that as stated by in the Wikipedia article

The second film reveals that a company named "IZON" is responsible for the development of the Cubes, and the third film indicates that their construction and operation has either been ordered or directly carried out by a non-descriptive "government". The films are alluded to be set in the future, however, one of the characters of Cube 2 mentions Muammar Gaddafi, who was alive at the time of filming.

Even in the third film Cube Zero which is prequel to CUBE and CUBE 2 it is mentioned that everyone in The Cube was facing a death sentence and was presented a choice: Go in the Cube with their memory completely erased or be sentenced to death. Only if a person signs the consent form will he or she then be placed in the cube.

However Eric discovers that there is no consent form in Cassandra's file (Cassandra is the main protagonist of Cube Zero and Eric is a watcher outside the cube). That meant that the consent is not always given for all prisoners.

In the end Jax shows Eric his consent form and says he had been put in the Cube not as a victim, but as an observer with his memory reprogrammed so he would have no memory about his past life.

In March 2011, Lionsgate announced it was considering an additional film in the series, tentatively titled Cube 3D (May be a sequel or Spinoff). This movie may explain about why people are kept in the cube without their consent and is there any other type of people in the cube except observer and victim.

You have to watch Cube and Cube Zero to get more of an idea of what's going on. Essentially, the Cube is owned by a rather powerful organization that uses it for "prisoners", or people that they want to get rid of. As we see in Cube Zero, when someone in the cube makes it out, they are then questioned about a few things. If they answer wrong they are destroyed.

So, we are left to assume that it's a trial of sorts, and once you make it out and face the final trial, you are free (in theory, we don't actually see this though).

I thought that perhaps all the people had died and that the cube represented purgatory. All the "inmates" were having to face, and reflect on, their own selves. Rennes says at the beginning "you have to escape yourselves". At the end, only the pure manage to go to heaven (the mentally handicapped guy is amoral), and the rest are condemned to death/hell/nothingness whatever. No matter what your religion, or whether you are atheist or agnostic, conclusions about moving (or not moving) on to the next life can be drawn.

The film-makers perhaps then decided to move the film in another direction in order to pump out a couple of sequels/prequels, but I won't watch them because I think the film itself is a masterpiece (not the acting necessarily).

Or... perhaps someone just got stoned while playing with a Rubik's Cube and had an idea for a movie about a maze in the shape of a cube.