You do if you want to be able to do your work from your dorm room/apartment/home - wherever you will be living while in school - else you will always have to go to the library to do your work, unless you are taking a desktop to school with you.
I took mine to class and used it to take notes. Most of us did this, although there were certainly a few who still liked the way of using a pen/pencil to take notes. Some also used iPads. How you take notes is totally up to you, however.
Sometimes I had work to do between classes - when I had a big gap between one class ended and another started - and it was handy having my laptop available to use rather than having to wait to get back home to do it.
Sometimes I used mine to review notes before class, or a test.
Sometimes there were library references I needed that I couldn't check out (because the library wouldn't allow that particular item to be checked out) or sometimes I just didn't want to check it out, so I used my laptop to type the info into.
So as you see from my examples, the portability factor was key for me - being able to take my work with me and use anywhere. I can't say if this will apply to you, obviously, since I don't know what your needs are. These are just examples of how you might need a laptop for you to think over.
Ultimately it honestly depends a lot on you. But I think if you don't get one before starting school, that after you get there you will find down the road that you wish you had bought one.