Robing has its origins in the 11th Century when Oxford and Cambridge first became universities to teach clergy Latin and to read the bible etc. The students wore black coats and differentiated between the colleges with a coloured hood.
Three hundred years later universities in Scotland, being St Andrews and Glasgow, followed suit and in the following century, Edinburgh. There were no new universities in the UK until the beginning of the 19th Century, when London and Durham became universities around 1830, and at that time academic wear started to evolve.
The coat was replaced by an open sleeved gown and the hood became less functional, and detached, to indicate the university rather than the college at Oxbridge.
In the 20th Century in excess of 100 universities were established in the United Kingdom, and whilst a lot of the black gowns used by Bachelor and Master awards were identical, the academic hood, just as it was in the 11th Century, identified the wearer's qualification and university.
What's an academic hood?
So your graduation robes are made up of three elements:
Gown - this is the coat style garment
Hood- this is not attached to the gown and will be unique to your university and level of award
Hat - This is either a Mortarboard or bonnet. Bonnets are for Doctoral awards.
What is the significance of the hood?
So the hood is not to protect you from the rain but is part of the traditional graduation attire.
The hood, the shape, colours and finishes are exclusive to the university, the level of award you gain and in some instances, even the faculty.