The three Cs of why you need to align your tyres are caution, cash and comfort since bad alignment can lead to accelerated or uneven tyre wear, it can compromise the tyre's grip on the road, especially in slippery conditions. In severe cases, it can even lead to a blowout, so alignment is necessary as a safety precaution.
Accelerated wear also means that you'll have to spend more to replace your tyres more frequently, so alignment is necessary from a financial standpoint as well. Dragging a misaligned vehicle will create scrubbing, causing resistance to the road surface and thereby decreasing vehicle fuel efficiency.
Finally, misaligned tyres can make your car's movements jerky or cause vibration, An alignment may help to improve this if there are no other issues behind the problem.