The excuse is to stimulate the economy by reviving inflation. However, the money has, for the most part, driven up the prices of financial assets instead of consumer and producer prices. The result has been a massive increase in the inequality of income, wealth, and opportunity.
The quantitative easing policy followed by central banks is based on belief in an economic relationship between inflation and GDP growth—the Phillips curve—that supply-side economics disproved during the Reagan administration. The belief in the Phillips curve persists, because supply-side economics was misrepresented by the financial media and neoliberal junk economics.