DEADThey’re to hide her square feet.
To quote Roald Dahl in The Witches
“Witches don't have any toes. They just have feet with square ends. A real witch will hide her ugly feet by squeezing them into pretty shoes, which they find extremely uncomfortable. Look very closely and you might see a real witch limping very slightly.”
Soph clearly thought it was safe to wear her witch shoes round at Brian’s.