Question about Wahoo Elemnt Roam

The Wahoo Elemnt is an independent GPS device. When it is switched on, it continuously determines its position (and the corresponding time) using GPS (among other things) satellite signals and records these measurements continuously. To do this, it must have a "view" of enough satellites; in tunnels, buildings, but also in dense forests or sometimes even in buildings, this is not always the case due to the system.
The display can be configured in many ways to show what can be derived from these (recorded) measurements. For example, the presumed speed with which the device is moved (position change per time). By actively starting and stopping a recording segment, the distance covered is displayed, and this is probably what you mean by KM performance.
Wahoo offers several additional sensors that can be included in the measurements. One of these is a speed sensor that measures the revolutions of the wheel. This "real" speed measurement is taken into account in the device for refinement and always improves the accuracy when the signal from the satellites is insufficient. Without this sensor, the device may show a more fluctuating speed, but otherwise works just as well.
This independence from other sensors can be seen as a particular strength of this type of bike computer, because it can be used immediately on any bike.

It seems particularly valuable to me that a simple map display with the current position is also available. The road and path network for this map display can rsp. must be downloaded free of charge from the Wahoo server via Bluetooth-connected smartphone (Wahoo ELEMNT app available for iOS and Android).

Only in tunnels would an additional speed sensor on the front wheel be a plus. Otherwise you don't need it.

it measures both by gps, so very accurately, plus the altitude metres as well