When do you know your car needs wheel alignment? And why should you maintain proper wheel alignment?

To answer the second half of the question, there are a number of reasons to maintain proper wheel alignment. Off the top of my head, accelerated tire wear and reduced fuel economy are my top two because they hit the wallet. Both improper camber and toe settings can reduce the effective footprint (the area of the tire that is in contact with the road) of the tires. This results in tires that wear out extremely quickly on either the inside or outside edge of the tread. Excessive toe also causes increased friction between the tire and the road because both tires are trying to go either toward each other (toe in) or away from each other (toe out) but they are connected to the car and can't. The extra power needed to overcome the constant skidding across the road surface causes reduced fuel economy. In addition to the financial aspect, there's safety. Sure, you eventually get used to a car that pulls slightly to one side but try driving a vehicle with the caster way out of spec (negative caster). If it's bad, it will require full concentration to make the constant steering corrections needed to keep it in a single lane at speed as it tries to dart to one side or the other for no apparent reason. Dealing with that on an extended trip is mentally exhausting and fatigue can lead to accidents. Long story short, a proper alignment can save you money on fuel and tires and also potentially save your life.