As you stated maybe for troubleshooting issues. Perhaps some people have their OS installed dorked up and have issues
 
I too run both the onboard sound and the nvidia GPU audio. Onboard is for my headset for Discord. I switch the main sounds between my monitor (HDMI) and the headset without any issues depending on where I want the game sounds or if I am watching a movie late at night

Are you talking about irq conflicts?

So it's just best practice thing?

Simply to avoid possible issues and not a carved in stone must be thing from what I uderstand.

Kernel memory is the memory used by the Windows kernel. It includes memory used by core components of Windows along with any device drivers.
 
IMO this is a Less is More situation.
 
Hey if it works, why fix it.  Maybe your pushing the last tiny bit of OC from the RIG and it crashes.
 
Even outdated audio driver tend to be dodgy & cause issues

Realtek drivers have a history of issues with other drivers
 
@jinihammerer
Why sometimes there are issues - best explained by these:
Using the Kernel Stack

(excerpt)
The size of the kernel-mode stack is limited to approximately three pages. Therefore, when passing data to internal routines, drivers cannot pass large amounts of data on the kernel stack.
To avoid running out of kernel-mode stack space, use the following design guidelines:

• Avoid making deeply nested calls from one internal driver routine to another, if each routine passes data on the kernel stack.
  
• Make sure that you limit the number of recursive calls that can occur, if you design a driver that has a recursive routine.
  

 
Running out of kernel stack space causes a fatal system error. Therefore, it is better for a driver to allocate system-space memory than to run out of kernel stack space. However, nonpaged pool is also a limited system resource.
 
Allocating System-Space Memory
(excerpt)

Drivers can use system-allocated space within their device extensions as global storage areas for device-specific information. Drivers can use only the kernel stack to pass small amounts of data to their internal routines. Some drivers have to allocate additional, larger amounts of system-space memory, typically for I/O buffers.


To allocate I/O buffer space, the best memory allocation routines to use are MmAllocateNonCachedMemory, MmAllocateContiguousMemorySpecifyCache, AllocateCommonBuffer (if the driver's device uses bus-master DMA or a system DMA controller's auto-initialize mode), or ExAllocatePoolWithTag.


Nonpaged pool typically becomes fragmented as the system runs, so a driver's DriverEntry routine should call these routines to set up any long-term I/O buffers the driver needs. Each of these routines, except ExAllocatePoolWithTag, allocates memory that is aligned on a processor-specific boundary (determined by the processor's data-cache-line size) to provide best performance.


Drivers should allocate I/O buffers as economically as possible, because nonpaged pool memory is a limited system resource. Typically, a driver should avoid calling these support routines repeatedly to request allocations of less than PAGE_SIZE because each allocation that is less than PAGE_SIZE also comes with a pool header that is used to internally manage the allocation. 

I have been using both Realtek High Definition Audio (Edit: always keep up to date with the latest, v10.0.17763.1 dated 9/14/2018) paired with Dolby Digital Plus and NVIDIA High Definition Audio without a single issue for two years. 
 
Edit2: Wait is this thread made primarily about PCIe audio cards in question of driver conflicts?
 
Edit3: Fixed spelling of Dobly to Dolby

You can use two sound cards at once, but you can't play the same audio source on both sound cards at the same time.
 
 
 
 

Manufacturer's say it to prevent excess troubleshooting requests for the few that would have driver conflicts. 

I've run 3 different sound devices / cards on a single system with equipment which says to uninstall the others... they work fine together. Windows 10 makes them play nice better than older versions of windows imo. 

I agree with you OP. Now.....back in the day (like Windows 95, 98, ME, XP) I remember all too well the trouble of things conflicting (and playing the card shuffle hoping to avoid IRQ conflicts while pulling your hair out). But since like Windows 7 and up that does not seem to be a problem anymore these days. What seems to be important to Windows now is only what is set as playback and what is set as recording in the taskbar. It seems that it ignores anything else. But I'm not a programmer nor did I make Windows, so I don't know what if any conflicts for a sound card still exist to this day.  But I can understand this from a trouble shooting point-of-view. Where its simply less margin for error to get rid of any audio generating device you don't want as your main and its corresponding drivers.