Strong authentication is required when connecting from external public networks
Strong authentication ensures that the person identified is in fact the person they claim to be. It makes impersonation, ID thefts, 'man in the middle' attacks, etc. much more difficult.
Strong authentication is a combination of 2 of the following 3 factors: something the user has in their possession (e.g. Digipass, dongle, ID card), something the user knows (e.g. a sufficiently complex password, a pin code, a tactile movement), or something the user is (e.g. a retina scan, fingerprint, vocal fingerprint).