Tanks were landed directly on the beach by landing craft on D-Day, in particular many of Hobart's 'funnies', the specialised vehicles like Sherman 'crab' mine flail tanks or Churchill AVREs, which were too cumbersome, heavy or just awkward to swim ashore. The DD was another specialisation, conceived with the Allied need to land as much materiel on relatively small beaches as quickly as possible. Landing craft posed large targets, and if they were hit, or the leading vehicles knocked out, they could foul up the beachhead and prevent other equipment coming ashore. The idea behind the DD was that individual vehicles would be harder to hit, would be less likely to clog the landing zone, and were not all bunched together.

Greenwood, D-Day to Victory