Tire Balancing adds weights to the wheel to balance it out. Variations in the tire and wheel manufacture can cause a slight imbalance. The valve stem, and now the tire pressure monitoring sensors in the tire, also play into the equation.
Even small differences can cause annoying vibrations at speed: the wheel is essentially bouncing a bit as it goes down the road. For example, at freeway speeds, an out of balance wheel can be slamming into the road 14 times a second. That’s annoying and can cause your tires to wear out more quickly.
If a front wheel’s out of balance you’ll feel the vibration through the steering wheel. When it’s a rear tire, you’ll feel the vibration through your seat. If you’re getting bad vibes from your vehicle, bring it in to see if it’s a balance issue or something else. You should balance your wheels whenever you get a new tire or remount a tire like when it’s been removed for a flat repair.