Why do some cells like myofibrils have multiple nuclei?
In the case of myofibrils this is the case because of the syncytial nature of that given structure. It means that in developing muscle tissue after a certain stage the cell divisions become somewhat different in that the final step, being separation of the daughter cells via invagination of cell membranes, does not happen. The nuclei though keep dividing after that. In the end what we have is a big membrane coated structure (basically a cell), with lots of nuclei in it.