Well, someone has to take on her resonsibilty of rearing the child. Let's say she recently had a baby and her and the father gave up the baby for adoption (and it's finalized).
Should she change her mind and decide she wants to be "mommy" again or play an important role in this child's life, it will be up to the new parents to grant her request. She is not responsible for the well-being, supporting with money or emotions, etc. But she also will not have a say in how this child is raised, where it should live, where it goes to school, religion, etc. If the parents decide to let her be involved in the child's life, it is totally up to them. She's basically just another person on the street.
Think it over carefully. It's permanent. (There may be more, but I haven't gotten there, yet. I was on the new parent end.)