A vehicle is said to be properly aligned when all suspension and steering components are sound, and when the tyre and wheel assemblies are geometrically set to run straight and true.
Automotive suspension system involves moving parts so wear of steering and suspension components is normal and is expected. As these components wear, alignment changes are normal, and have to be adjusted to bring the settings back into the specification range.
Alignment is also sensitive to ride height. As the springs get old and sag or bushings deform over time, the vehicle rides lower and misalignment occurs. This changes the load distribution across the tyre and affects the rate of tread wear (i.e., a heavily loaded area will wear more quickly than a lightly loaded area). Whatever the cause of misalignment, the result is that the tyres do not roll as straight as they should. This causes scuffing; uneven, and (sometimes) rapid tread wear.