New Delhi, Aug. 21 -- We all care about the interest rates that central banks set, but what about the $19 trillion of bonds and other assets held by the biggest among them? Those constitute a more esoteric, but equally consequential, element of monetary policy that has lately transcended the wonky to become politically fractious.

Supporters credit bond purchase programmes with stimulating the economy in times of need by lowering longer-term borrowing costs and tamping down market disruptions. Opponents blame them for widening inequality, among other things.

What becomes of these bonds is critical to governments, businesses and households because they influence the cost of money. And because detractors say they are a big reason why trust...