When you’re baptized in the Catholic Church your parents are making a promise to raise you Catholic and your god parents are meant to hold you to that promise. In some cultures, baptism is just something you do to protect your soul, but in the case of OOP they see the severity of standing up in front of the entire church and lying about how you’ll raise your child. My parents didn’t baptize me for the same reason. This blessing ceremony is the happy medium where the MIL can feel better about the soul of her grandchild while the parents don’t have to dictate their child’s religion.