They help to hold the space for permanent teeth. Your child may need one if he / she loses the teeth prematurely, before the permanent tooth is ready to erupt . If a primary teeth is lost early, adult teeth can erupt into the empty space instead of where they should be ultimately leading to crowding and difficulty in speech.

Overcrowding of teeth due to faulty eruption of the permanent teeth in prematurely extracted teeth

Prevention of overcrowding by different types of space maitainers

Fixed functional space maintainer in a 4 year old girl