The asterisk is for the situation when the current buffer is a
read-only buffer—a buffer that cannot be modified. If
insert-buffer is called when the current buffer is read-only, a
message to this effect is printed in the echo area and the terminal
may beep or blink at you; you will not be permitted to insert anything
into current buffer. The asterisk does not need to be followed by a
newline to separate it from the next argument.