Why not just ask them?
I'm sure they'll probably have a justifiable reason or theory behind it but I'd be more inclined to expect that on a fine art course rather than a graphics course.
It could be just something as menial as presenting your research in your final show and to make sure they're larger scale and all uniform with the other students.
Personally, I don't like using them as I find them too big and unwieldy to sit with one on my lap and transport without a portfolio.
Not good for 'blank page syndrome' either.
Also, unless you have access to an A3 scanner it makes it difficult to digitise any images and bring them into a computer.
I tend to use cheap copy paper for my roughs and then an A4 layout pad for refining them.
Unless they had a reasonable argument for using one and you don't get marked down then I'd use what I was comfortable with.
Just because they're tutors doesn't make them always right.
When I was at collage Mac's came along but we were marked down for using them in out final pieces as they thought they were a bit of a gimmick and considered it cheating.
Work that one out!
