They're all fairly specific – in terms of "writing" apps, Bear is for notes, Craft is for documents, Ulysses is for blog posts, Slugline is for video scripts, MindNode is for brainstorming, Agenda is for date-based notes (I use it to track stories).
Reeder is ingest, Instapaper is longform, GoodLinks is a tagged archive and also great widgets/Shortcuts support.
Structured is daily routines, Fantastical is work