Could anyone please explain to me why I have been put into a plaster cast up to my knee after breaking a bone in my foot that leads to my little toe and sustaining swelling to my ankle?
Probably because having most of your leg in plaster will make you in general less mobile, and encourage you to put your leg up more, which will help you heal faster. It probably isn't strictly necessary for setting the healing bone in the correct alignment. But overall you will probably do better with it than just a foot cast - you might still manage a wiggle of the ankle with only the foot plastered!