For many people, who don't have patios or decking to treat, buying 5 litres of something when they only need a tiny amount is wasteful. The Polycell product is cheaper and convenient.
Actually, the best stuff to use is Boracol 5RH, which includes a specific anti-mould chemical as well as an anti-algae chemical. I have 20 years' experience of using it on the teak decks of my last sailboat, as well as on the canvas sprayhood of my current boat. It's very good - a couple of applications annually keep the nasty stuff at bay.