It's for peace of mind most times. Having a Lambda sensor readout where you can see it is a preventitive measure against leaning-out fuel. Lots of very basic reasons why this can happen to an already well carburettored engine... fuel blockage, surge, tank vent indaquate, clip falls of the needle etc.
When testing/developing a new exhaust systems the readout is handy to see exactly what is going on in real time on the track. We often put the Lambda readout next to the tacho and point a GoPro that can see the track, the tacho and the readout at the same time. Often see quite different results to what we get on the accelerator dyno but similar to the water brake dyno.
Pumper Carbs can give misleading readings on the accelerator dyno because so much fuel lies around on the floor of the port, it can't be atomised or used in an 8 second rev up run. Also, if running open velocity stacks the dyno room is acting like a huge airbox with little turbulence. On the the track the velocity stacks are dealing with +200kmh wind turbulence that can affect their operation.
It's just another handy tool to have actually.