It is not necessary, but it is advisable.
Adoption records from another country can be in a different language. When your child wants to get his/her driver’s license or other legal document, having a birth certificate in another language, makes getting the new document difficult, if not impossible. If you lost the adoption records or birth certificate from another country, it may be hard to replace it. A re-adoption here will allow you to access copies of these important documents from the court and your state’s department of vital records.
If you want to change your child’s name, a re-adoption would be the time to do that as well.
Some states do not recognize a foreign adoption decree. Illinois recognizes the foreign decree, but if you were to move, and move to a state that does not recognize it, that can be problematic. Or if your child were to considered an heir because of the adoption, but the probate estate was in a state that did not recognize foreign adoption decrees, your child may not be considered an heir.
For the most part, an adoption in Illinois of a child already adopted in another country is a fairly painless process. You will need to have the decree translated to English, but all of the scrutiny has already been completed by the other country. The parents’ rights have already been considered, whether there was a consent or a termination. There are always exceptions to the rule, but typically, a re-adoption is done fairly quickly and painlessly in Illinois.