When you use the Shotwell Viewer to explore images in a whole directory, you will suddenly notice that you can not delete the image you are currently viewing using the Shotwell Viewer.

Actually, you have to look for the correct image in your file manager and have to delete it there.

Web browsers – like the default for Ubuntu: Firefox – are currently using an extra windows to manage downloads.

It's often necessary to switch between to windows quickly. (For following an instruction, Checking up something in the Internet, …)
If the windows, you're working in primary, is not fully maximized, there is a thing that sucks every time:
You're writing something in an editor, whose window isn't fully maximized. You want to search someting in google, so you click into the window of the browser, which is visible behind the text editor. The editor window dissappears, so if you've finished the search, you have to find the program in the starter to make it visible again and to continue to work.
The last step coasts a lot of time, and is annoying.