First off, Dynamic DNS, also known as DDNS, is a service that will instantly update your IP address (the A or AAAA record) if the host (device) changes it.
When your IP address’s lease ends, your ISP (Internet service provider) can change it automatically.
You can use DDNS to ensure that the device will remain accessible if you utilize it as a server. Otherwise, you won’t be able to reach the new IP address or determine it from a distance.
Without Dynamic DNS, if you are operating a monitoring server with a camera at home and you have been viewing the video from a distance, the connection will break the instant the ISP changes the IP address, and you won’t be able to see anything.