If teeth are not in contact with another tooth they will move towards the open space. This can create problems with your bite. If one tooth starts to move it can disrupt the alignment of your entire mouth. Out of place teeth can be very difficult to correct and require expensive, time consuming procedures such as an orthodontics to correct.
This is why it is important to restore any missing teeth as soon as possible with either a Implant, Bridge, or Partial Denture. You can discuss with your dentist which options would be best for you.