You all got me curious, so I did a search for the uses of bisulfite and bicarbonate in developers. My collection is not as big as Ian Grant's, but it is pretty good. In almost all cases, the only purpose is to control pH. Bisulfite is particularly used in pyro stock solutions to keep the pyro acid. It is also used to lower the pH in many fine grain developers. It occurs in many litho developers, where it is sometimes used with paraformaldehyde to produce a developing agent. Some stranger applications of bisulfite also show up, however. The most notable is in Rodinal type developers, where its pH control possibilities should be overwhelmed by the hydroxide. It also shows up in a lot of photofinishing developers, where it is used with carbonate and bromide. The probable design use in that case is to stabilize the pH. Some paper developers use bisulfite when additional warmth is needed.
Bicarbonate is rare. It is usually used as either a very low pH alkali or with carbonate as a buffer. FX55 is a curious variation. It contains both bisulfite and bicarbonate with carbonate. That would be a very strange buffer, so Crawley must have had something else in mind.