There are more reasons than just aesthetic appeal for correcting crooked, crowded or protruding teeth — they also carry greater risks to oral health. For example, teeth that are crowded and overlapping are difficult (if not impossible) to clean properly and are at increased risk of decay. Further, protruding front teeth are at greater risk of accidental damage, as well as potentially causing problems with speech (and kissing).
The way that your teeth fit together when you bite is important too. If the upper and lower jaw fit together poorly (malocclusion) then this can cause stress to your temporomandibular joint, potentially causing pain and headaches.