Pitching in on the community one. I love simple low-maintenance clicking games with nice lil' forums mainly because of the forum.
Playing other things like Flight Rising, I spend far less time on the forums since I have a higher focus on the game itself. To the point of sometimes forgetting the forum is even a thing I can kill some time on. Here though I come to click the few buttons for the game part, and then spend the rest of my time browsing the forum.
"How is that relevant to the game?"
Whenever I get bored of FR, I simply don't go there. It's a solo game I can put off and deal with it half a year from now. Maybe two years. Or five. And then boom it's gone.
A game that goes hand in hand with a forum has more of a draw to come back and check how's the forum doing. And then, obviously, you also play the game itself.

To me it hardly feels like "extremely engaging petsite" was the main reason for this games existence. Feels more like Animal Crossing in that sense - socialize a little, do your daily tasks and then come back another day. Not the best comparison but it's the first chill thing I could think of. So I can certainly see myself coming back for a long time. The biggest appeal IS the low-maintenance imo.
But then again I'm also a busy adult so I can imagine younger people with more free time will feel otherwise. Or people who aren't keen on socializing.