This is the reasoning. The Federal Reserve reasons that if stocks go up, wealthy folks will borrow against their stocks and buy things other than investments and the economy will strengthen, justifying the higher stock prices. This is not Keynesian economics or any type of normal monetary policy. This is the equivalent of an NFL coach making up a new play on the side lines during a game and telling the quarterback to run it. The benefit to stock buyers is that the Federal Reserve has been successful in moving stock prices higher. Who knows how long they can continue doing it?
There is a big risk in taking a catastrophic loss all in one day. Stock volume is extremely light and is dominated by high frequency trading firms. Company insiders are selling into recent strength in record numbers. The slightest economic mishap will cause the bottom to fall out of any thin market, especially one that has been artificially stimulated.