The purpose of creating a Pastel Blue How to Pet a Dog Poster is to educate people on the proper way to approach and pet dogs, ensuring their safety and comfort. The poster includes information on how to read a dog's body language, what areas to avoid, and the appropriate way to pet them. It also emphasizes the importance of asking the owner for permission before interacting with their dog. The poster aims to promote responsible and respectful behavior towards dogs while also preventing potential accidents or aggression.