The Watling Street Hypothesis underlies a number of suggestions for the site. This scenario provided by Nathan Ross outlines a variant of the original.
1. Paulinus brought his whole force down to London, rather than doing Webster's supposed 'cavalry dash'. The latter phase of the campaign therefore begins from London, rather than heading off into the Midlands.
2. He needed a position to withdraw to after evacuating London, as he was escorting a large number of civilian refugees. This place needed to be in allied territory, within scouting distance of the rebel advance, on a communication route to potential reinforcements from the west and the north, and close to a defensible position. St Albans meets all these requirements - and why would Tacitus have mentioned it if it wasn't significant?
3. Remaining close to St Albans allowed Paulinus to block or intercept attempts by the rebels to move back towards their own territory via the Iknield Way, which they would have needed to do at some point in order to plant winter crops and avoid famine the following year.
4. Once the rebels approached St Albans, Paulinus pulled back once again and took up a position in the Chilterns, either blocking the route directly (at Dunstable) or threating the rebels from the west (at Tring). Either location would have allowed him to preserve his lines of communication with reinforcements from the west or north. Sited on chalk uplands and on a watershed, both sites would have provided a dry battlefield with water supply for both the Britons (from one direction) and the Romans (on the other), and an easy route of approach for the Britons after they sacked St Albans (which is only 10-12 miles away)
5. By positioning himself at either location, Paulinus would have left Boudica's rebels no option but to confront him directly.