By default, windows in BeOS operating systems are modeless, meaning that you can click on other windows besides the one with the focus. Occasionally, there is a need to force the user to make a decision before moving on. This is an appropriate time to use a modal window. The need is far less often than what many would believe, however. Most of the time the only reason to use a modal window would be if not doing so would make potential for the user to lose data and there is nothing that can be done to the code architecture to prevent it. This follows the "no restrictions for no good reason" way of thinking mentioned above -- modal windows place restrictions on what windows he can or can't click on.