Yeah, I was going to respond with a grouchy "Circles are the most intuitive thing in the world" non-self-aware response :P, but you're right. A lot of people didn't know what to do with them! As is frequently the case, my mind can work differently than other people, and Circles were "YES! THIS IS WHAT I'VE BEEN LOOKING FOR!" instant-love for me. Heck, I've been trying to do it with facebook for years and I just can't - it has a "Groups" or something functionality, but it's burried and difficult to use and complete pain to setup.
Whereas for me, circles were amazing to both send and receive. I have a group of friends that are interested in photography, and a group of friends interested in computers, and a group for astronomy and philosophy and bicycling and whatnot. So I can place people and or they can subscribe to what they want to see from me.
Similarly, people would create a circle of say interesting photographers, and you can then share that circle and copy that circle and modify that circle and subscribe to that circle.
What ended up happening for me is that after a bit of time, I had a daily feed that was just... way better than any algorithm or AI or ML or privacy-invasive company ever managed to accomplish. Just high quality stuff all around. But also my friends and followers also received better quality stuff from me, as I could tailor who gets to see what.
I also liked the interface at the time, seemed to actually use my wide screen monitor, be clean, and relatively uninvasive (open FB on a 32" monitor and you barely get more useful text than on the 6" phone to this day)