"The real value in using Zotero is building a searchable, sortable, annotated, and organized database of literature. That's what will really pay off 10 years down the line." (Sebastian Karcher).
Here are examples of how some people use it:
Zotero is of course a database for bibliographic references, but you can also create standalone notes for ideas or concepts.
To create a note, menu "File" -> New note or button.
Notes can be tagged, linked to references through the "related" tab (related and tags functions are at the bottom of the note), put in collections. Zotero search functions allow to retrieve them easily.
"I take notes in many different places, but I ultimately store them in Zotero with the relevant references. The benefit is that you can use the search and tagging features for notes too. For instance, I use tags to keep track of good quotes I might want to use, and I copy and paste core arguments into notes so that I don’t have to go through a whole article again." (Mark Dingemanse, Radboud University Nijmegen)