Also why, exactly, are they buying the exact assets that they are buying (govt. debt, high-yield bonds, etc..) and why not others (e.g. stocks or put money into startups)? And then, what happens if a debtor pays back its debt? Is that money consequently getting "erased" again (just like it's been created)? What happens if a debtor defaults on its debt? Does that money then just stay in the economy, impossible to drain out? What is the general expectation of the central banks? What percentage of the debt is expected to default and how much is expected to be paid back?
And specifically in the case of central banks buying govt. debt: Are central banks considered "easier" creditors than the public? What would happen if a country defaults on a loan given by a central bank? Would the central bank then go ahead and seize and liquidate assets of the country under a bankruptcy procedure to pay off the debt (like it would be standard procedure for individuals and companies)?
So the aim is to take advantage of this transient fluctuation and the way the ripple propagates.
But even if in a perfect (or future) world, everybody reprices instantly all the goods relatively to the new amount of available currency, there is still an effect which is how is distributed the newly "created" currency: who get the new shiny coins? So this is equivalent (if the repricing is done) to a kind of global instantaneous tax-and-subsidies. They tax everybody by the percentage of currency created (relative to the total existing amount), and the lucky ones receiving the fresh money are getting thus a subsidy.