What advantage does this Sophos product have over others that do the same, some Open Source, like RAR, 7-zip etc.
Very probably none at all! As I said, I haven't researched it much yet.
I was previously unaware that zip programs now offered the ability to properly encrypt rather than merely password a file. Like many other paid-for products, there will almost certainly be free alternatives with the same features and as long as they are from known, trusted players in the market, I'd recommend those every time.