E.g. you want to access some server or services at your home, but your ISP is changing your ip address every 24 hours. With a DDNS service you can always use the same dns name, only the a/aaaa records of these will be updated when you get a new ip address from your isp.
There are some uses cases, e.g. you have many external devices that are not having reliable internet connections and are not smart.
The real enterprise use case is rather a full dns service for domains with some additional services (that will be available soon on BlaBlaDNS ;)
