I think it just gives Catherine the freedom to be "on" and have a clear look that she is very comfortable with and we all recognize. Then she can go home and be herself and be free to dress however she wants. Having a uniform gives her clear boundaries.Paradyning I know a lot of people are not crazy about her choices but they always fit her beautifully and you can easily see how it reduces anxiety about constantly figuring out new outfits if you've already narrowed the scope.
I think the coat dresses are genius. They are simple, they can be one bold colour so she can easily be seen in a crowd by onlookers and security, they are feminine and yet have an element of armour about them as they are modest and buttoned up so she never has to wonder if anyone or a camera is peering down her top.
People like familiarity, we love the visual comfort of the Queen in her own uniform: coat, hat, brooch, handbag, sensible but smart shoes. In Balmoral: corgis, kilt, twin set, pearls, headscarf and country coat. Maybe Catherine is quite comfortable not being exciting but being respected and loved. She seems quite a naturally shy person and yet she has learnt to be very comfortable and charming while doing walkabouts.