It is your choice if you want to preorder. Some like to do it to save time in line, or know their favorites. I believe you can go to your reservation online and there will be an option to preorder, but have never done it myself.
I personally don't usually know what I will be in the mood for in advance, so just order when I get there. Last time we ended up moving CP brft since they changed hours and it was no longer PPO, so were not that hungry even for late lunch, so just got soup and dessert(of course!) So it is up to you - potato leek soup is great by the way!