Manual invalidation is crucial for several reasons. Firstly, it helps in improving deliverability rates by reducing the chances of emails being sent to non-existent or inactive addresses. This, in turn, can enhance your sender reputation with ISPs (Internet Service Providers). Secondly, it aids in enhancing engagement metrics such as open rates and click-through rates by focusing on recipients who are more likely to interact with your emails. Lastly, it can save costs, as many email service providers charge based on the number of emails sent or the size of the email list.