A cosmetic dentist can do quite a bit to improve the overall appearance of your teeth and your smile; this can mean filling in small gaps between teeth, making undersized teeth appear larger, and the like. However, an orthodontist is concerned about the actual alignment of your teeth and not just their appearance. If you're thinking of visiting a dental professional to address problems you have with your teeth and their appearance, note when and why it's good to see an actual orthodontist rather than a cosmetic dentist.
An overbite is not just unsightly; it can also be dangerous to your teeth. If your bottom teeth push up against the back of your top teeth, this can cause tooth erosion. Those bottom teeth might also be resting on the roof of the mouth, near the back of your top teeth, and this might cause irritation to the skin. A severe overbite might also mean more pressure on the lower lip from the top teeth, also irritating the skin. A cosmetic dentist can sometimes file down the upper teeth or otherwise reduce their overall size so the overbite isn't as noticeable, but this doesn't protect the inside of the mouth or your teeth from this potential damage.
When teeth are very crowded, they may be rubbing against each other and causing erosion, as mentioned above. A cosmetic dentist might put veneers or bonding over the front of the teeth to make them seem aligned, but this too doesn't correct any problems with potential erosion, as the teeth will still be overcrowded. If those teeth are pressing against each other, they may also be putting pressure on the tooth roots; over many years, this might cause the teeth to actually become loose. Having them properly aligned is the better option.
Large gaps between teeth
Some gaps between teeth can be filled in with bonding or veneers, or adding a cap over a tooth might make it bigger and the gap less noticeable. However, note that these artificial agents may be more prone to cracking and breaking when used to fill in a gap, as there is nothing to support them. You may need to have this procedure done over and over again when you choose this option versus having the teeth actually straightened. This will also ensure that protruding teeth, which often cause gaps, are properly aligned so that you have little risk of having it get broken or cutting the inside of your mouth.