It is well known fact that all deciduous teeth don't fall (exfoliate) at the same time and all permanent teeth come (erupt) at a single time.
So, if a child's tooth (deciduous) falls or needs extraction at an early age, there is a chance of space loss. Space loss is due to the migration of neighboring tooth/teeth towards the vacant/lost tooth place. This causes insufficient room for the corresponding permanent tooth to erupt completely, resulting in malposition of the tooth.
So, space maintainers can be used in the spaces when required. They prevent migration of neighboring teeth and the space is preserved for permanent tooth.