If you feel that you want to certaing clothing when doing spells or rituals then you can do so. I have seen some wiccans, pagans or shamans where different clothings while performing a ritual or spell, because it feels comfortable and respectful to the spirits or to what ever they are doing in their path.
But most people just where what they ever feel comfortable. :)
Wearing a robe is really like using any other tool in Magick and the purpose is to help connect you with your energy better as well as hone the energy more. It is not needed, though it helps certain people.