The thing is, most people do check "Copy Items to Library." If it isn't checked, then Photos may not know where your pictures are. If it is not checked, then when you add pictures to Photos, it just tries to remember where each picture came from. If anything moves, Photos will lose track of it. If the "Copy Items to Library" is checked, Photos makes a copy to store inside its Library package so that the photos and the data base that goes with them are always together. Of course, this means there are two copies of the pictures. But then you can delete the folder they came from (or move it to another disk) and Photos will work just find. This disconnect from a folder containing the originals is what usually causes the "cannot be found" problem.