Most CPR certifications are valid for 2 years. Recertification is important because it teaches new techniques and guidelines that may save someone's life.
CPR guidelines are frequently updated to reflect a more effective technique. As of 2015, CPR is done by first doing compressions, then opening the airway and administering breaths. Before, it was thought that opening the airway was more important. Renewing a CPR certification allows people to save more lives than ever before.
Many companies and organizations that require their employees to be CPR certified offer certification classes or pay for their employees to be recertified when their certification expires.