The only reasons I can think to do this are 1) the onboard RAM has errors 2) the socketed RAMs specs are so different from the on board that it creates errors.
Yes, 1) - in case the onboard RAM gets malfunctioning.
And you may not want upgrade it, but to just leave in the board and make BIOS use only the soldered RAM.
Resoldering a couple of resistors on the board seems like not a big deal to make BIOS forget the soldered RAM.
As I am planning to buy a laptop with partially soldered RAM, I want to ensure
the possibility of dealing with it successfully.
The only reasons I can think to do this are 1) the onboard RAM has errors 2) the socketed RAMs specs are so different from the on board that it creates errors.