When dealing with browser security, there is a concept called “the line of death“. This concept means that a user can only trust content that appears within the browser’s address bar or above, and nothing below that line (there is an excellent article from Eric Lawrence who is a Chrome developer explaining this in detail). What that means is that users can click on content above that line safely, but not below since the content appearing below the line may be fake or modified by the attacker. However, it is clear that the rest of the browser UI including menus, settings sections, about box, etc. are static and should be static and safe (unless modified by extensions).
The same concept would apply to mobile apps – part of the UI that are static should be safe as well, although it is harder to tell the static and non static parts apart. This leads add to the issue at hand – what happens when the static parts of the app have hyperlinks that don’t use HTTPS? A user of the app would normally trust those links but if they are on a hostile network, clicking on a plain HTTP link would in fact expose them to a potential MITM attack either via DNS hijacking or MITM interception. That means that if they are using a network where the attacker controls the DNS or the network connection itself, these links can be easily hijacked. You can easily image a scenario, where an attacker blocks WhatsApp or Facebook traffic but redirects users who use the HTTP versions to their own malicious site.
On the other hand, when HTTPS is used for these links, the mobile browser will check if SSL certificates are being served on that link, and whether they are signed by a real CA.