Focus was Mozilla's way of offering Tracking Protection on iOS in some form without slowing down the performance of its main Firefox for iOS app. In addition to blocking ads within the Focus browser itself, Focus also provided iOS content blockers, which ironically (due to iOS content blockers being exclusive to Safari) allowed Focus users to block ads in Safari but not in Firefox for iOS at the time.[2]
Focus migrated to WKWebView in 2017 after iOS 11 bridged WKWebView's feature gap, allowing Focus to retain its Tracking Protection feature through the transition.[3] With Firefox and Focus both on WKWebView, Mozilla then ported Tracking Protection to Firefox for iOS in 2018.[4] Tracking Protection improved Firefox for iOS, but could never be as comprehensive as the Safari-exclusive content blockers.
By that time, Mozilla had already launched Focus for Android in 2017[4] even though Firefox for Android already had the ability to block ads more comprehensively with extensions such as uBlock Origin. As a privacy-focused secondary browser with a one-tap "erase session" feature, Focus was a niche app that turned out to be well-received by users, and Mozilla probably did not have a good reason to discontinue it even after Firefox for iOS gained Tracking Protection.
[1] https://news.ycombinator.com/item?id=12977855
[2] https://blog.mozilla.org/en/products/firefox/introducing-fir...
[3] https://github.com/mozilla-mobile/focus-ios/issues/274
[4] https://blog.mozilla.org/en/products/firefox/latest-firefox-...
[5] https://blog.mozilla.org/en/products/firefox/firefox-focus-n...