Wheel width should be simply a byproduct of the tires you choose to run. If you want to run 35x12.5R17 tires like the Pitbull's I have then the manufacturer suggests a 10" wide wheel. I run a 17x9 wheel and all it does is make the tire bulge out 1/2" on both the inner and outer sides. I don't mind this as my wheels will have a less chance of getting scrapped up on rocks as my sidewalls stick out beyond my wheel.
To give you a contrast the 4x4 guys in Hawaii like to run 35x12.5R22 tires with 22x14 wheels to make the tires look like they are stretched over the wheel. IMO this is not only stupid looking but it also increases the chances of your tires braking a seal putting other drivers at risk.
My point is wheel width should match what tire you want to use.
So what do wider rims do???
i know it pushes the tire out but...
is it for looks? A softer or harder ride? Worse tire wear???