A car likely needs a wheel alignment if its tires are unevenly worn. Vibrating steering wheels or those that are not centered are also indicators, as is a car drifting to one side during use.
Even if these symptoms are not present, a car may still need alignment if the owner's manual recommends that tires be aligned at a specific interval that coincides with the car's most-recent alignment. Worn tires by themselves are not necessarily indicative of a car that needs its wheels aligned; the wear needs to be uneven. With some tire models, the uneven wear is most noticeable on the outside.
Alignment becomes necessary when a car's suspension gets worse, as with worn or overly stretched springs. Simply bumping a curb may be enough to knock suspension off-base, meaning the wheels sit at uneven angles. Aligning wheels ensures they sit straight.
Less tire wear is one of the top advantages of getting an alignment because tires are expensive at most retailers. An alignment generally costs less than half of the price of a single tire and should be part of regular car maintenance. Tires that are in sync also get better gas mileage because they have less resistance.