There are several reasons why it is recommended that you replace your missing tooth once the extraction site has healed:

For each missing tooth, you lose about 10% of your chewing ability and put more force on the remaining teeth.

As soon as one is missing, the surrounding teeth start to drift into the empty space which can lead to a change in your bite, gum disease and possible tooth decay.

After an extraction, the bone that supports the teeth begins to shrink over time and your face adjusts with it. 

Even a missing back tooth can cause your mouth to shift, potentially making your face look older. No one wants to look and feel older than they are.