When you're slacking off in an office setting that requires a computer and have a window up for display purposes to simulate work. Often requires Alt+tab to display your Slackground when browsing non-work related content.
For example: Spread Sheets, Important looking emails, Memos, ect.
Oh crap Boss Roger just walked by! Good thing I had my Slackground displayed so he did not see the NSFW pictures I was looking at.