The terrain that you hunt, the type of animal that you plan to hunt, and the distance that you're comfortable shooting will determine if you "need" a range finder.
For example, if you're only going to sit in a tree stand hunting whitetail deer, and you have the time to step off and either mark or memorize distances and won't be shooting long range, then you'd be just fine without one. BUT if you're going on a spot and stalk/open terrain hunt out west and you have the ability to take long range shots, then you'd be better off with one.