Tires wear how the car points
If the alignment is off, a brand new set feathers and cups within a few thousand miles. You paid for tires and threw away half the tread. An alignment protects the set.
Front and back
Front-end alignment is the common one; the rear matters too on a lot of cars. We check and set both the front and back so all four corners track straight.
Rotate on schedule
Tires wear at different rates front to rear. A rotation on schedule evens it out and stretches the set. We do it while the car is up for the next oil change.
