But why is it a good idea to have a saver web view outside of Safari when Safari remains wide open?

Maybe they think that people are aware that the web is generally less safe and act accordingly? And they don‘t want someone to smuggle that unsafeness into apps? (The homescreen webapps are more or less just collateral damage if you take on this view.) That’s the only somewhat plausible explanation I can think of and it doesn’t make all that much sense.