Good question. As far as I remember, the SSILD technique was first published by the name "A very mysterious technique" in Chinese by CosmicIron, so as the name implies we don't quite know why it works. I think it provides a good mix of relaxation and focus, and switching from one sense to another may boost one's attentions - it prevents us from uncounsciously drifting away, so to speak. But then again, if you do a wbtb and keep your focus on your body, this too could serve as an anchor. So, there is some similarity and overlap, but not quite.