This logic will require that at least one Contact has been added to an Account before an associated Opportunity can be marked as Closed Won. Having a Contact for all Closed Won Opportunities has several potential use cases:
Salesforce’s Einstein Activity Capture, which syncs calendar events and emails to Salesforce, will only work for Contacts in Salesforce.
Your Customer Success or post-sales team may need to know which stakeholders to reach out to in order to start onboarding
Your Marketing team may want to pull a list of current customers from Salesforce to include in their marketing emails.