There are plenty of useful/interesting things, e.g. reactive lights, more proactive climate control, controlling ventilation on demand, presence based interactions etc. Some of this can be achieved with dumb solutions, e.g. humidity controlled switch etc. But when you start integrating devices together they can coordinate to do much more complicated and "smart" things.

Does everybody want to do that? No. How far do modern systems go towards achieving any of that? Ehh, IMO not very far unless you put a lot of effort into it. Do the UIs of modern systems really make setting up these sorts of complex interactions up accessible for the average person? Nope. Are there glaring privacy and security concerns? Yep. There are lots of perfectly valid criticisms, especially of "smart" homes as they are, but there are certainly lots of possibilities.