Personally I preorder packs that I know I'll definitely want (such as Cats & Dogs) for a couple reasons. I'm awful with saving money so if I preorder it then I can get that out the way and know I'll not be scraping together money in November. Also, if I don't preorder it I'm constantly thinking about when it's going to get released and counting down the days. If I preorder, I can pretty much put it to the back of my mind and know that when it comes out it'll just be downloaded automatically. That being said, if I have any doubts that I want a pack I'll wait for the reviews and I only preorder if I'm 110% sure I'll be getting it regardless.
This only really goes for the sims EPs though as that's the only thing I've ever preordered without bonuses. I've preordered a couple other games that have had offers and bonuses (Life Is Strange: Before The Storm had a small bonus of an extra outfit choice and South Park: The Fractured But Whole gave me the first game totally free for preordering) so it makes sense then if you get something extra for it.