The Sigfox Repeater can be used to extend the reach of the signal at positions where the normal network coverage are insufficient. This is intended for e.g. indoor applications, where there is no signal deep inside the building, but there is coverage by the windows.
More information is available at Adeunis's support pages:
Notice: Limitation when using repeaters: When you are using a repeater, you will lose the ability to downlink to your device (you cannot change the device settings). Therefore it is very important that you set the correct data transmission interval before you deploy your device.