Batteries and capacitors would serve different functions. Capacitors primarily isolate each individual chip and subsystem on a PCB from high frequency power fluctuations when digital circuits switch or larger loads turn on or off. You would still need to use capacitors for that. The purpose of the batteries would be to support high loads on the order of minutes that exceed the actual wall plug capacity to deliver electricity. I am thinking specifically of the stove linked in your sibling comment, which uses lithium batteries to provide sustained bursts of power to boil a pot of water in tens of seconds without exceeding the power ratings of the wall plug.

It is the same function on different time scales. If you had a big enough capacitor, you could achieve the same thing. Not coincidentally, the capacitors in PSUs are huge, although not battery sized in terms of capacity. The purpose of the capacitors in the PSU is to keep things powered during a power outage to allow for a UPS to switch to battery. The technical term for this is PSU hold up time.

I consider smoothing out high frequency power fluctuations and providing power over a period of minutes that exceeds the capacity of the wall plug to be conceptually two different functions, even if they have similarities.

You’re right that a large enough capacitor could do that, and I’ve worked with high voltage supercapacitor systems which can provide tens of watts for minutes, but the cost is so high that lithium batteries typically make more sense.