I'm pretty sure it's to kind of hide stuff that you want accessible but not have to be seen @Faith-and-Hope. I always thought of it like a way of covering the plot of a movie but having it there for others to see. I really have no idea. However I used it recently I put a really dark heavy image up and it just dominated the page and made it heavy. I didn't want to lose the image so I hid it in the spoiler. Now when you visit that page you don't see that image until you hit spoiler. However on the forum it often happens acccidently.
