Complex sentence: When used in gutters the cable is not meant to keep the gutters free of ice or snow, but only to provide a free path for the melted water to get off the roof and down the downspout or drain piping.
Split into simpler sentences: When used in gutters the cable is NOT meant to keep the gutters free of ice and/or snow. The purpose is to keep a free path for the melted water to get off the roof and down the downspout or drain piping.