Making decisions, planning for the future and even getting on the same page about anything related to a child can be difficult. Add in the fact that you and your child's parent are no longer romantically involved, and that can make coming to a decision more difficult. Parenting plans exist to take out some of the stress and guesswork when it comes to raising your child.
This would be in addition to a child custody decision in hopes of laying out expectations and responsibilities of each parent. Parenting plans can involve all sorts of details, including pick up and drop off times and other specifics that help in making the process of co-parenting more smooth and with less uncertainty and disagreements between parents. They can't regulate every aspect of a child custody agreement, but they can help in many respects.