Third-party applications can add this ability, such as the open-source com0com, freeware HW VSP3, or the commercial Virtual Serial Port Driver.
[1] This technique can be used either to extend the capabilities of software that cannot be updated to use newer communication technologies, such as by transmitting serial data over modern networks, or to achieve data flows that are not normally possible due to software limitations, such as splitting serial port output.
For instance, a GPS device which outputs location data to a PCs serial port may be of interest to multiple applications at once.
This allows software intended to interface with a device over a local physical serial port to instead communicate at long distance.
The drivers for a software-implemented modem create a virtual serial port for communication with the host operating system, since the modem is implemented entirely in the device driver and therefore there is no point where the serial data would be sent to the physical card.