I usually give posts about my own work a short shelf life - 2 or 3 weeks, and then remove the entire thread.
But the thread will have suggestions and comments from other QT members that I want to think over more before revising the manuscript or query, which I usually feel I need to look away from for a bit, and come back to it with fresh eyes. So I want to save the file for future reference.
I used to copy and paste the whole thread into a Word doc, which meant shrinking or deleting large avatar photos to save space, not realizing that clicking on PRINT does not automatically print out a paper copy. It offers a view the thread text with nothing but the posts people wrote -- no pictures, no peripheral QT images, etc.
From that point, you can save and/or print out the plain thread. I prefer to save the document. It's searchable with CTRL-F just like any other web page.
So now I know how handy the PRINT function is for saving a thread as a file. Just passing it on.