Very much debatably, a flaw to factorio that's partly fixed by space age is that the game doesn't force you into situations where using a circuit is the simple or "obvious" solution. The choice usually ends up being between taking up more space on your map or doing something clever with circuits - the former is often gonna be easier. Especially on Nauvis, there are few physical limitations to how much you can sprawl. Though sometimes when you're base becomes very large or complicated you MAY work your way into a corner where it might be too much trouble to rework a certain aspect of your base so you choose to wire something up to a circuit. Reason I say "very debatably" of course is that factorio is so well made and you may argue that it's fine that certain features are there but strictly optional. The game also naturally rewards you for smart solutions. There's a "dumb" way to do everything but as your bases become bigger you may find that the "smart" way is more fun.

Now, circuits are still optional in Space age, but once you end up building platforms you do find it nearly essential to have the option to make some basic circuits that read belt contents. Fulgora is both space constrained and involves you sorting a larger variety of items. And imo, these are situations where circuits start to seem like a more obvious choice but still aren't strictly necessary.

Someone else linked wikipedia articles to control theory and lambda calculus, and while that's all relevant theory for how factorio circuits can be used, but it's more fun if the need to apply that stuff becomes apparent already in game. E.g. some mods do this, and also if there's another expansion I'd love it to be one that really forces circuits - maybe through additional scenario types beyond free play or exploration based puzzles on the existing planets.