Many subwoofers have a stereo connection, which it is often a good idea to use via a Y-split subwoofer cable for optimal performance.
The reason is that the bass unit can be equipped with dual voice coil, or just with multiple devices. Most splitter filters in subwoofers therefore perform best if both inputs are used.
Therefore, it is often advantageous to use a split cable, even if the original signal is a mono signal. In practice, the effect is most often that the woofer plays louder than when just using the one input, and we always recommend using a subwoofer split cable if the subwoofer has such a stereo input or no mono or LFE input is directly specified.