3. Why Use Wheel Spacers?
Appearance. You want to push the wheels out for a better stance and more aggressive look. This is totally subjective to your personal tastes. And we can't tell you what to like...
Clearance. Many wheels are not compatible with big brake kits. The spoke of the wheel will scrape the new brake caliper and you need a wheel spacer to push the wheel spoke away from the caliper.
Correction. You may have bought a set of wheels that don't have the correct offset for your car. The offset may be too high, resulting in the wheels sitting too far inward. This is both ugly and incorrect as the tire can now rub on the inside wheel well, or on suspension components, etc. A wheel spacer will push the wheel out and 'correct' the offset.
Handling. By spacing the wheels further apart, you can make the car more stable and corner better. You can gain a similar effect as adding a wider wheel without the added weight and expense.