In an ideal world, nobody would need to be adopted, but this is not an ideal world. Adoption parties cannot prevent a child being taken into care, but they can help take them out of care. They give 'hard-to-place' children a greater chance of placement. They tackle some endemic problems in the adoption process.
Abuse and neglect have obvious emotional, physical and psychological costs. The monetary impact is more challenging to analyse and anyone who does so risks vitriolic accusations of being as cold, insentient and azoic as the calculator used to make their estimates.