What are the reasons for and against wearing a bicycle helmet?
For: If you fall on top of your head, it will protect it (also side impacts if wearing a full-face helmet while riding downhill mountain bike)

No, this is not an overly simple reply. It's what it's designed for, it's what it does well and it's why you should wear one.

Feel free to edit with documentation about head protection, but I think the number of people on here (myself first) who can say "I fell and my helmet saved my head" are worth any research.