This function is only useful for applications which retrieve and poll libusb's file descriptors in their own main loop (The more advanced option).
Ordinarily, libusb's event handler needs to be called into at specific moments in time (in addition to times when there is activity on the file descriptor set). The usual approach is to use libusb_get_next_timeout to learn about when the next timeout occurs, and to adjust your poll()/select() timeout accordingly so that you can make a call into the library at that time.
Some platforms supported by libusb do not come with this baggage - any events relevant to timing will be represented by activity on the file descriptor set, and libusb_get_next_timeout will always return 0. This function allows you to detect whether you are running on such a platform.