Not sure where it should fit in the current UI? Oh just throw it in a modal, restricting the user's ability to interact with anything else on the page, requiring an extra click to close the modal.
I'm sure there are good times to use them, but typically I find they are restrictive.
They are particularly ineffective for protecting the user against errors, except on systems which use them very sparingly (such as in the web, thanks to browsers adopting non-modal bars for most warnings and search actions.) We older netizens remember the nightmare that was every other web page throwing a modal alert(); that blocked the browser on all the open tabs.)
[1] https://en.wikipedia.org/wiki/Modal_window#Design
[2] https://en.wikipedia.org/wiki/Mode_(computer_interface)#Asse...
Simply put, blocking the UI is hostile.
On the other hand, I don't know much about them so I don't even get the idea that I could use them in a UI design.