It probably comes from someone actually threatening to sue over a "stolen idea" when two queries crossed an agent's desk with the same-ish plot, one of which was rejected and one of which was accepted and eventually published.
If you could see how often I finish drafting a book, only to pick one off the shelf that has, not only a similar plot, but similar names, themes, symbols, etc. I nearly threw a book across the room when, on top of the similar plot and same color-coding I used for different groups, one of the characters had the exact health problem I had given the equivalent character in my book. This on a book I had just started querying.
Somebody in a similar situation who had been querying a while might accuse an agent or publisher of "stealing their work." That's why that's there.