The significance of the 3 dates
79 AD is when the Verulamium Forum and Basilica were re-dedicated after the destruction of the town by Boudicca. They were impressive buildings and indicated the wealth of the 3rd largest town in Roman Britain and also showed that it had a degree of self government.
793 was when Offa, king of Mercia visited the Shrine of Alban and re-founded the monastic settlement and Abbey. The town then developed around the Abbey as pilgrims came to pay homage to the saint. St. Albans now began on its new site and the old Roman town gradually disappeared.
1868 saw the completion of the main line from London St. Pancras to the midlands via St. Albans. Thereafter the town expanded quite rapidly as the commuter link to London meant that goods and people could reach the capital in under an hour.