In commit 7e1f86d4 monitor support was added to randr. At this time it seemed to be reasonable not to have more than one (virtual) monitor on a particular physical display. The code was never changed since.
Nowadays, extremely large displays exists (4k displays, ultra-wide displays). In some use cases it makes sense to split these large physical displays into multiple virtual monitors. An example are ultra-wide screens that can be split into 2 monitors. The change in this commit makes this work.
Besides that, removing a monitor in a function that is called "RRMonitorAdd" is bad practice and causes unexpected behaviour.
More information and discussion can be found at https://gitlab.gnome.org/GNOME/gtk/-/issues/2013#note_1564528 .
Are there possible impact to people that rely on this feature?
Potentially, that change breaks scripts that adds a monitor to a display where already a monitor exists. In my opinion it is very unlikely that such situations exists. There may be scripts that add a single monitor to a screen so that the monitor has a name different from the implicit default monitor name that is assigned if no user defined monitor is added to a screen. This still works as before with my change. Besides that, it is always possible to delete existing monitors with randr/xrandr.