Just wondering, why are unmanned rovers almost always using six wheels (adding more weight) as opposed to four wheels? Is it for added traction, redundancy, balance, or a combination of these factors?
Short answer: More wheels results in greater stability, allowing the rover to traverse more varied types of terrain. It also provides redundancy, in case one of the wheels breaks, the rover is not completely crippled.
The Mars rovers all use the same design, called the Rocker-Bogie. This is a special design which allows the rover to traverse many different terrains while keeping the body of the rover stable. Since the wheels are not connected to each other, it also allows the two sides of the rover to go on different slopes simultaneously.
This design allows Curiosity to tilt 45 degrees in any direction without flipping over (though automatic sensors prevent it from going over 30).
In addition, each wheel can be equipped with its own motor. This allows the rover to rotate in place and to continue functioning even if one of the wheels is damaged. For example, when the Mars Exploration Rover, Spirit, broke one of its front wheels, the other wheels were able to provide enough power to keep it going.
There are other designs which differ from Rocker-Bogie suspension, but most of these also use six wheels. (Such as the proposed Russian Marsokhod rover which never actually launched.). Having more wheels increases traction and stability, resulting in a safer rover overall. You could make a design with four wheels, but this would be less stable. Alternatively, you could put 8 or 10 wheels on, but this would drastically increase the cost and weight of the rover. Therefore, 6 wheels is a good middle ground that provides lots of traction and stability without being too heavy.
The rocker-bogie suspension has its origins in a 1969 study by M. G. Bekker, 'Introduction to Terrain-Vehicle Systems' which I haven't been able to find online.
By 1994, NASA had designed a series of rover concepts named 'Rocky'. They started with an articulated 6-wheeled rover. By the 4th iteration (yes, Rocky IV), the 6-wheel rocker bogie design was established. The 6th iteration would lead to the Sojourner design which flew in 1997 as part of the Pathfinder mission.
A discussion of rocker-bogie vs other designs can be found in 'Sojourner: An Insider's View of the Mars Pathfinder Mission' by A. Mishkin.