The idea was to give Namibia (then German Southwest Africa) access to the Zambezi and the Indian Ocean. But what was the point? Victoria Falls is downstream from the Namibian portion of the Zambezi, and there are no locks...
This German news article gives as primary reason access to the Zambesi in order to connect German Southwest Africa with their East African holdings later on. Apparently the plan was to acquire the territory which became Rhodesia and create one continuous German controlled strip spanning the whole width of Africa. That would have also given them access to the Indian ocean from what is today Namibia.
Cecil Rhodes endeavors and his company's close connection to the UK made that impossible.