Lets imagine you are in your shower early one winters morning, the light is on because its dark outside. Your partner or children switch the toaster on and it has an electrical fault. This might trip out the RCD that protects the power circuit for the toaster, but for some reason it’s the same RCD that protects the light in your bathroom. (See picture below) Your lights go out, you grope around in the pitch black to turn off the shower and get out, you trip on the edge of the shower tray / bath side, or slip on the floor, cant see anything to catch hold of and fall over. You might get out of the shower/bath room and fall down the stairs because you couldn't see the top step.
This used to be called “Nuisance Tripping” in the 2008 Regulations and all reference to it were deleted in the 2011 revision. However, most electricians miss out section 314.1 at the front of the 2011 Regulations that still requires “Every installation shall be divided into circuits, as necessary to (i) avoid danger and minimize inconvenience in the event of a fault”.