In many cases, a) the unique id's are in read-only parts of the flash and b) the system firmware won't work if it can't read a unique id that matches a (signed) stored ID somewhere in its system.

In many cases, this isn't for anything user-facing. Unique ID chips can be very useful for detecting factory-overrun counterfeits ("oh, your $widget does this notable, nonfunctional behavior? Where did you purchase it, perchance?").