The first ever Google mobile friendly ranking boost launched before April 21, 2015, it was launched back when it was announced for apps that used app indexing. Apps that had deployed any app indexing received a ranking boost. Now, apps that use the app indexing API get an additional ranking boost (well, not currently iOS apps).
Google's Michael Fink, the product manager of the Google Search Console, said at SMX Israel that you should not build an app just to get a ranking boost. He said, only build the app if you can provide a better user experience for users.
Kalman Labovitz posted this on Twitter, I was there, Michael Fink did say that:
Honestly, the main reason I built the Search Engine Roundtable iOS & Android apps was to report back to you all about how the ranking boost matters and share my stats. Of course, many love the UI, speed, push notifications, etc of the app. So there is an improvement for the user. But my main reason was to test the ranking changes and share it with you all.
It is not uncommon for Google to tell you to think about the user before the search rankings. But can't we have the best of both worlds? I think so. Why give the ranking boost or tell us about the ranking boost if you don't want to influence people to build apps for it?
Forum discussion at Twitter.