[2] In the United States and Canada a block trade is usually at least 10,000 shares of a stock or $100,000 of bonds but in practice significantly larger.
By definition, the stake was large enough to affect supply and demand causing a market impact.
Unlike large public offerings, for which it often takes months to prepare the necessary documentation, block trades are usually carried out at short notice and closed quickly.
Further, the fact that a large, well-informed money manager wants to sell (or perhaps buy) a large position in a particular security may connote future price movements (i.e., the money manager may have an informational advantage); by taking the opposite side of the transaction, the broker-dealer runs the risk of "adverse selection".
[4] Block trading is a useful measure for analysts in order to assess where institutional investors are pricing a stock, because in a merger or acquisition, a bid needs to "clear the market" (i.e. enough shareholders need to tender), it is most useful to see at what prices large blocks of stock are trading.