Read the uses section.

It doesn’t make sense that silicon tetrachloride is a byproduct of purifying silicon. Why would you leave the silicon atom attached to the chlorine if the silicon atom is what you are after. The ratio makes sense - there’s one silicon atom and four chlorine atoms, but the statement that it’s a waste product is strange to me as it’s not. It is a intermediate and you do have to house it, and you can leak it and that’s not great (but as they state in the article you linked it’s not stable around water so I wouldn’t lose a huge amount of sleep over it’s pollution.