Consider a phase to earth fault on a piece of metal equipment connected to a 3 phase floating neutral system. Should a second fault to earth occur on another phase then a phase to phase voltage will appear between the metal parts of the equipment and ground.On the other hand a grounded neutral will reduce that voltage by sqrt 3
Plus it is a requirement of UK wiring Regulations. Small sets serving one single circuit such as you find on construction sites need not be independently earthed as the likelyhood of a double fault situation is low.
Paul has also highlighted dangers which put floating systems beyond the Pale for general use.
