I can't see the reason to have them at Bank as anyone travelling from
Bank DLR should have touched in somehere on the tube (not necessarily
the case at Canning Town or Stratford)

The validators are for people who didn't enter the system at a tube
station
but want to use prepay for their DLR journey.

How do people get to the validators without either arriving at the
station by tube train, or entering at Bank tube station? I can only
think it's people who arrive by DLR and immediately want to retrace
their steps.

Aren't there lifts for the mobility impaired that go from street level down
to the DLR level? Those presumably bypass the gateline and might be a
additional reason for there being validators there.