My (headless) Raspberry Pi 3B (boots from SanDisk Ultra Fit 64GB USB stick) will not boot at all without the Logitech universal keyboard/mouse dongle plugged in one of the USB ports. With it plugged in the system boots and works like charm. Without it, nothing. I do not get even the early boot messages via the serial connection (UART), seem like nothing is read from the USB stick (no lights flickering). For more information, see: background info from Raspberry forum.
Perhaps somebody here could shed some light into this?