In message .com, MIG
writes

But at Bank, you must have either started an Oyster journey already (on
LU or DLR) or else just come through, or about to leave through, the
barrier line at the entrance. So what are the deep-level Oyster
readers for?

Possibly for (disabled) people who have come straight down from the
street on the DLR lift. There was a discussion of other possible reasons
here last November - Google should reveal details.

--
Paul Terry