What are tooth space maintainers really nessisary?

Permanent teeth: If baby teeth are lost too soon, the permanent teeth may not erupt in the proper position. A space maintainer will help reduce the chance of orthodontic problems later.

In children: If you mean a maintainer in a child who loses a tooth early, then possibly yes. It's important to maintain space for adult teeth and if it is not coming in for a while then leaving the space open until it does is important. A dentist can see by x-ray. See a qualified dentist. If you mean a retainer after ortho then that is only way to guarantee teeth not relapsing.