If a baby tooth is lost prematurely, you may need a space maintainer to "save" the space. What does that mean? Baby teeth are there for a reason. One key reason is that they save space for the permanent tooth, which will erupt into its position when the baby tooth is lost normally.
If a primary tooth (baby or milk tooth), has to be removed early due to say, an abscess, or is knocked out in some kind of trauma, a space maintainer may be recommended to save the space. If the space is not preserved, the other teeth may drift causing difficult to treat crowding and orthodontic problems. If it is a front tooth then you don't need a space maintainer. However, you can place a Pediatric Partial to replace the teeth for cosmetic reasons.
Now "Spacers" may be in there for a while, but they are not permanent. They are removed when the new tooth (usually a bicuspid) erupts or the abutment teeth get loose.