Bosch powertube batteries come in various physical sizes. Generally speaking, the larger the battery capacity, the longer the powertube battery is physically. This part extends the physical length of a 500Wh powertube battery to make it as long as a 625Wh power tube battery. Naturally it makes no difference to the actual battery itself, so your bike won’t go any further

Some bikes have space for a 625Wh battery, but only come with a 500Wh battery. Therefore the 500Wh battery uses this spacer to fit inside the frame.