It is useful if you start thinking about products whose prices depend on the prices of other things. Suppose that I'm a SPY trader. I need to know where a bunc

It is useful if you start thinking about products whose prices depend on the prices of other things. Suppose that I'm a SPY trader. I need to know where a bunch of other things are trading in order to figure out where I should be quoting SPY. If I only know those prices on a second stale basis then I can't get my hedge off and so I can't quote SPY as tightly. I lose, investors lose, and investment banks win because they're better adapted to trading where prices aren't clear.