I worked at Jaycar and helped them set up their operation on the west side of Australia back in the early 90's.
I (and others) pushed for the development of these fuel mixture monitor kits.
They do operate as a volt meter of sorts that displays the voltage of the o2 sensor., which is quite logical because that is its output.(what did you expect?)
A normal digital voltmeter averages the display which can make viewing the output of the sensor inacurate.
The o2 kit shows it as it happens.
Later kits (which I was not involved with) I believe had a pic chip in them ..I dont know how good/or bad they work.