You should replace missing teeth for a number of reasons. Your appearance is one reason. Another is that the gap left by a missing tooth can mean greater strain is put on the teeth at either side.
A gap can also mean your ‘bite’ is affected, because the teeth next to the space can lean into the gap and alter the way the upper and lower teeth bite together. This can then lead to food getting packed into the gap, which causes both decay and gum disease.