This might be a silly question - but why do some universities want a photo with your application?!

It is unusual for universities to ask for photos so early, though it is likely you will need to provide at least one photo for the university you end up going to. The photo is needed for your registration or nus card, which is likely to be the main form of student ID you carry round. Universities often ask for photos early so they can have your registration cards ready when you arrive - though now does seem very early.

It's also worth bringing along a few passport photos with you when you arrive, as you may need them for other forms of id such as a library card or a registration card for sports clubs or socities.