That's what I thought at first, which is why I was disappointed when it didn't work for me straight away with just the 'keyboard' USB plugged in.
Perhaps the keyboard needs the extra bit of power from the second USB as I think the downstream USB ports on the monitor might be powered by the upstream USB cables from each of the computers. The Dell user guide isn't as detailed as I would like, but it does say that the downstream USB ports (other than a separate fast charging port on the back) have a maximum power consumption of 4.5W
I've only had the configuration working for a few hours, so I'm sure there's more to learn. At the moment for example, the keyboard switches to another (non-default) profile when I switch between PCs - but that could be due to my lack of full understanding of the Utility Engine default settings!