Sometimes there are extra goodies if you preorder - f.ex. XCom 2 had those, I followed the released information and gameplay videos (the developers invited some people to their studios, and allowed them to record and publish the gameplay, if I recall, or maybe just gave them the beta copies to play), thus having quite good idea of what I'm buying, and only preordered it about a week before release date, when it had to be finished, with at most only manufacturing and boxing the discs etc. left to do (and I don't regret it at all).
Or in case of WoW, character boost if you want to use it right now for something (I boosted my warlock to 90 with WoD's, ran some LFR to gear up, and got the green fire with FoS when it was still available - in hindsight, I probably would be able to get to 90 in time, and obviously I wouldn't preorder WoD or maybe even buy it at all, but nevertheless, some reasons to preorder it were there).
Those two are the only things I preordered, so in the end it's 50/50 for me.