They would be introducing money into the economy. The people that had their bonds purchased by the Fed would be holding money that didnt previously exist. This

They would be introducing money into the economy. The people that had their bonds purchased by the Fed would be holding money that didnt previously exist. This dilutes the value of an individual dollar. Just like dilution of any asset. In the context of currencies this is called inflation. Typically the whole point is for private persons to invest into the real economy, even though it is riskier, because these other new ventures are going to try to put it to productive use, instead of any party just parking the money in a bank account since now it is clear it will lose value just parked.