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 impersonations, 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 from the user (e.g. a retina scan, fingerprint, vocal fingerprint).