These are piercings in the rim of the glans and are generally placed as but some people do just get one. These piercings are said to have been started by Jewish medical students who felt that they had lost sensation through circumcision and wanted to replace that. Which is not such a random idea as often the area around a genital piercing will be more sensitive.
The soft tissue of the glans makes this piercing more likely to reject if it is not placed correctly or cared for properly during healing.
Although this was designed for the wearer more than the partner these can also create stimulation for the partner during intercourse.
Dydoe piercings have not been hugely popular through the years but are not a rare piercing by any means. In the late 1980’s when jewellery was hard to get rings were often placed in these for the healing period but barbells became the preferred jewellery and less likely to catch.