Hi,

If you are creating a new customer the email is not required.  But if a customer is assigned as a related partner for a user, then the field email become  as required field.

Go to customers menu , create a customer, you can create a customer without email. Now go to Users and create a user , when creating user , a partner will also get created.  Now go to customers and open that partner form, now for that partner/customer the email will be required field.

Thanks