I don't really know the original reason as to why GIFs need to be spoilerized (new word invention right there). I joined the forum team after this rule was implemented, I believe; so all any of us really know are what a few people claim the reasons are. Quite frankly, it doesn't matter why we have the rule, we just do. We have lots of laws and rules in society that just plain shouldn't have to be there as they are common sense (apparently we need laws to tell us not to shoot each other), but we still have them in place.
That being said, I'll say this: I personally do not like looking at random GIFs. When I go to a thread to read something, or look for a trade, or respond to a question, I don't want to see some flashing, changing, Miley Cyrus, Spongebob, delicious food, cute puppies, adorable kittens...you get the idea. I just want to see the text, and if I want to see the possibly cute GIF, I'll click the spoiler tag and take my chances. Maybe this rule will change in the future (like rules often do) but for now we'll most likely keep it.
Perhaps this rule started out being in place for those with epilepsy, but now I like to think it's in place to help keep the forums clean and nice looking.
luto: