For a while I used Obsidian to contain all kinds of notes I take: timeless notes, project notes and journal notes. Now that I have discovered Agenda I keep my project and journal notes there instead.
Agenda is great for notes, that are transitory in nature. I am a person who needs a writing surface to think and Agenda makes it easy to keep a somewhat ordered writing environment. One thing I really like about it: It makes it very easy to keep periodic notes, without me having to maintain a custom system.