Your www host record (www.mydomain.com) for your domain is the only record that has a valid destination.
Your @ host record (mydomain.com) points nowhere valid, so the domain name cannot be resolved.
There really is no difference functionality-wise. The user can access the site no matter if you choose to use either or both. Most sites use both in some way (e.g. resolving the www record to the @ record or vice versa).