I imagine she needs it because if there is anything out of place or she commits a "fashion faux-pas" according to the Daily Mail's list of fashion faux-pas, which includes things like wrinkled dresses, VPL, body hair, too much fake tan, too little fake tan, showing too much flesh, not showing enough flesh, choosing too bright a colour, choosing too dull a colour, having cellulite on any area of the body, then she will be slated in the most widely read online newspaper including close ups of the offending item.
She may be rich and beautiful, but I imagine she is still a human being and likely to feel hurt by such negative scrutiny and therefore does her best to avoid it. Also, she earns money by being the face of a beauty company, and may lose her contract if she appears as anything less than perfect at all times.