If you are missing a tooth, it is important that you get it replaced for a number of reasons.  As well as affecting how your smile looks, a missing tooth can put strain on the other teeth as your chewing action changes. 

The extra space can also encourage your remaining teeth to grow crookedly. This will exacerbate the strain on your teeth and cause more problems in the longer term. Food can gather in the gap and attract the bacteria that cause halitosis and tooth decay.

By filling in the gap, your teeth become easier to clean.  In some cases, a missing tooth can also affect your speech.