Twitter unceremoniously shut off access to most third-party apps nine days ago—a crude evening failure followed by rushed-out policy changes to suggest, if nothing else, that it was at least intentional. Most imagined a late-night Elon Musk "decision" which allowed for no forethought, finesse or planning; some imagined a hasty effort to cover for a breakage that the company no longer has the on-staff expertise to repair. Mitchell Clark writes that third-party clients made the site what it is today; the creator of the summarily-executed Twitteriffic app is more acerbic, pointing out that their app was first to use the term "Tweet" and to use the bluebird logo that was ultimately appropriated by Twitter itself.
Third-party apps have had a massive impact on how we use smartphone apps in general, not just Twitter. A client called Tweetie is widely credited for inventing the pull-to-refresh interaction that's become almost ubiquitous throughout iOS and Android for refreshing all sorts of feeds. Even if you haven't heard of Tweetie before, you may have used it; in 2010, Twitter acquired it and made it the official iPhone client. In 2015, the company also hired a developer of a different third-party client to improve its Android app.