well displaying text box doesnt have to focus the text box to be of benefit. for a simple text message, such as one for a reminder, just having simple text box pop up with a short word on the taskbar will work. so in this case, it's preferrable for the text box to not interupt user activity and steal focus from another program.
for example, you have a scheduled text box to remind you of a tv show at 8pm. meanwhile, your writing an email or playing a game. and when that text box shows up, you don't want it to steal focus from your currect task--not even temporarily, as it will interupt your keystrokes, etc.