Many iconic features of Twitter have been created over the years by listening and watching what people who use Twitter do with it and then working to make it easier and better for them — we still do this today.
The Hashtag at Ten Years Young
Biz Stone

….and by third part developers who Twitter then crushed and locked out. Twitter apps used to be a playground for iPhone development, and lots of new features came out them — like tweet translations and photo uploads. Thankfully, a few of those apps still survive and continue to show the official Twitter app how it should be done. (Twitter bought one of them, too, before the API freeze…)