The best part of giving someone external access to your vault is that it's free of charge. No need to add additional seats to your subscription.
The role 'external members' is perfect when you need data from your client or contact. With this type of role, the person can add their sensitive data in a specific folder in your vault. They can also see the other vault items from that specific folder.
External viewers can only view content of a specific folder that you shared with them. This means that they can't add new data.