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In the days of analog TV broadcasting, only one video+audio program could be carried in the RF bandwidth of a TV channel (real channel) so there was never a need to 'label' the video+audio program... we all just called it by the RF channel number.
Now, with digital broadcasting, each station sends over 19 million bits of data per second over the real RF channel. That data can be used to send one high quality video+audio program, or as most broadcaster choose to do, several video+audio programs can be sent by a process somewhat like that used to share your Internet connection with one or more computers, each running one or more programs that access the Internet at the same time. The virtual channel number is used to help you and your TV tuner sort out which data to display as video and audio. The bottom line is, a real channel often carries many unrelated data streams that need to be labeled or coded so they can be sorted out on the receiving end.
The broadcasters also wanted a way to retain the legacy channel ID... Many moved their operations off of one real channel (that they had operated on for decades in many cases) as the result of operating in both digital and analog modes on separate channels during the transition from analog only to digital only broadcasting. Most stations continued running their digital operations on the new real channel after the old analog transmitters were shut down at the end of the transition period... The virtual channel number system allows their branding to continue using a long established 'Channel Number'.
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