Hi.., & welcome..!!!
The idea was to be able to check fuel level in the tank not being used at the moment. (W/O disturbing fuel flow) (you could say it eliminates the need for two gauges)
"...why not just one or the other?"
No matter what.., you have to have a "valve"
However.., there's an aftermarket elect. valve available that uses the dash switch to change the tank feed & the fuel level reading on the gauge simultaneously.