Modal notification windows are usually designed like this:
An example taken from Stack Exchange chat
A common feature of these modal notification windows is that there is a button roughly saying "Okay, I understood", and nothing else that you can close the window, including the typical "cross button on the top right corner". The implication seems to be that I'm forced to show that I understood it (otherwise it'll block the space and hence whatever I intended to do) even when I might not, or that I don't really care and I just want to close it.
What's the design purpose of these kind of window with the only option of "Okay, I understood"?