Central banks may occasionally intervene with purchases or sales from official reserves as a way of sending a signal to markets of its view of appropriate monetary policy. But intervention mainly is done with the goal of taking volatility out of the market when there is a run on the currency. But instead of holding cash, central banks hold U.S. treasury bills or bonds. There are large liquid markets for these securities and they can easily be converted to cash. Funds deposited with a financial institution for a specific term.
CDs are the retail equivalent of money market paper and short-term bonds.
The usual reason is that voting rights will vary among classes. The origin of this arrangement generally is that an entrepreneurial founder raised funds by selling stock but wanted to retain voting control of the company. One way of achieving this was to have voting and non-voting shares, also called restricted shares. Another was to have multiple voting stock, where each share had more votes than the company's standard common stock. Some of these arrangements provided certain dividend guarantees to classes of stock with reduced voting rights.