They were buying debt to massage the interest rate, and they were entitled to do that because their basic task was to ensure price stability, which in 2020 meant that the economy must be prevented from sliding into deflation. Avoiding deflation meant stimulating demand by all means. For the central bank, that meant holding interest rates down. Once again it came down to financial markets. As far as anyone could figure out, QE worked by driving government bond prices up and yields down.
