When Ubuntu is run on a laptop, the brightness control allows to simply define the desired level of brightness. This control is not available on a desktop, probably because energy consumption is not considered a problem there.
However, I find it extremely convenient to be able to change the brightness anywhere, if only to reduce eyestrain. I can't find a way to configure Ubuntu to display this control on a desktop.
There is a ton of Gnome Extensions and other things allowing to do something similar, all with their limitations, and extra complexity.
Isn't there a way to force Ubuntu to display the brightness control on a desktop ?
Your desktop computer is connected to a monitor, which is a stand-alone device. Typically the desktop (or more specifically: its graphics adapter) has no control over the brightness / background illumination setting of the monitor. The laptop is a more closely integrated system where the display controller comes with an interface that is accessible by software.
It would be possible to just dim the colors in software (or give them a tint, which is what the "Night" setting does) - but it won't be possible to actually reduce the amount of light the monitor's background illumination emits. And that is what you would have to dim in order to save power.
It's actually quite surprising that there is no standard interface between monitor and computer to allow such control.