Some subscribers - residential or business customers - may need a private IP-address. There are some specific situations, for example, for connecting several computers in the network or for getting access to a computer, or for email servers, security cameras, home automation, printers, and remote access systems. These kinds of technologies are particularly related to port forwarding and may require a static IP-address.
Some service providers offer a static IP-address for free upon a request. While many ISPs sell it for an extra fee for those exceptional cases, thus making revenue from the offering.