Here is more info on spacers
http://www.jeeprenegadeforum.com/fo...railhawk-edition/12058-wheels-tire-specs.html
Most cars from factory, the wheels sit inside of the fender. People who use spacers like to have the wheels flush with the fender. For cosmetic reasons.
If you get the wrong offset on a new set of wheels. Spacers helps correct it. Every car has a different stock offset, so one wheel might not fit the same on two different cars. Always research what offset you would like before purchasing new wheels.
Any spacers under 1" or 25mm or 2.5cm is installed by sandwiching it between your hub/rotor and the wheel. It utilizes the original lug bolts and nuts. Extended lug bolts may be needed if you are using spacers under 1".
Spacers 1" and larger will utilize a new set of lug bolts. So you would mount the spacers onto the hub/rotor using the original lug bolts and nuts. You may need to cut the lug bolts shorter to insure that when the wheels in mounted to the spacer, it is mounted flush. Once the spacer is mounted to the hub/rotor, you mount the wheel to the spacers new set of lug bolts. Make sure you get the pattern you wish to use to match the wheels you wish to use on the spacers new set of lug bolts.
If you are using spacers 1" and larger you can also change the bolt pattern to utilize the wheel you wish to use. The Renegades bolt pattern is 5x110. There isn't many wheels out there for us to use because this is a uncommon size in the USA. So using a spacer that bolts onto the 5x110 and changes the new bolt pattern to 5x114 will open up the available wheels choices we can make. Especially the offroad style wheels that are available to the Jeep Wranglers.
Wheel spacers can let you use wider wheels with wider tires. Without a spacer you may experience the wheel or tire rubbing your strut tube or spring perch. In the case of the Renegade, the spacer moves the wheel out away from the spring perch allowing for a slightly larger tire diameter. Without the spacer the tire may rub the bottom of the spring perch.
For offroading people prefer the look of a wider stance. The wider stance also gives the car better balance by lowering the center of gravity. Helps when you are at all kinds of weird angles when you are offroading. So the wheels usually stick out half way out of the fenders. Look at any Jeep Wranglers for an example.
Most offroading enthusiasts also recommend that for every inch you lift your vehicle, you should widen it as well. This is to do with the center of gravity again. When you lift the vehicle up 1" then the center of gravity also moves up. You can compensate it by moving the wheels out 1".