No you dont have to preorder. However if you do preorder you don't have to risk the game getting sold out before you get it, and you can beat the lines.
Pre-ordering will increase your chances for Limited Edition - check out the video on the front page if you're unsure what this would include, Limited Edition is the same price as a normal copy of Crysis 2 but with a ton of added benefits - but of course, they are 'limited'