It appears that a slip is to prevent light shining through your clothing and revealing, as Diana found out, your underpants. Another reason is to protect the body from the itchiness of Wool. And another reason is static cling. So a slip protects you from exposure, wool or static cling.
So there are good reasons to wear a slip and equally good reasons not to reveal that you are wearing one.
But for some Brave Women there is a Gang in San Miguel that throws caution to the wind and comes out wearing their slips as outwear. Perhaps they are Madonna wannabes or perhaps they like the look in people’s faces as the slip slips by. There is no shame for this group as they parade in the Jardin in their slips. They might use a little bit of embrodiery to disguise what they are wearing but a slip is a slip and for men something quite naughty.