[saagarjha]

A notable reason to use UIWebView is that it allows for changing network requests (for example, to add special headers). WKWebView does not because it runs out-of-process. So some of the web views you're seeing might actually be UIWebViews, depending on the context you're seeing them in.

[untog]

WKWebView now does allow that, in a roundabout way, through WKURLSchemeHandler.

[saagarjha]

It seems like this can only be used to support URL schemes "that WebKit doesn't know how to handle"?

[untog]

Yes, you create your own. In my example of the Instagram activity feed you'd point the view at instagram://activity-view or something like that in order to intercept the request and send a custom response. You could use the same method to "proxy" through to HTTP.