Is there any practical usage in real devices for connecting batteries in parallel? Or is it more a theoretical construct? It seems to me that only batteries in series are used nowdays.
Batteries placed in series have a higher voltage that is the voltages are summed. Batteries placed in parallel can withstand a higher load. Thus engineers match series parallel configurations to meet voltage and load requirements for everything from the 9 volt in your smoke detector to multi kilowatt uninterruptable power supplies.
The first consideration is the voltage requirements for the load. Some loads (motors in particular) won't run at all unless there is sufficient voltage.
The next consideration is the demand (magnitude of the load) on the battery. There are two aspects of demand- how much current the load needs to operate and how much energy the load will consume (power x time) for the expected duration of the load.
Connecting identical batteries in series increases the voltage, but not the current delivering capacity of the batteries. Connecting batteries in parallel increases the current available from the batteries, but not the voltage.
If you simply need more voltage, you can connect the batteries in series.
If you need more current but not more voltage (over voltage can damage some loads) you can do several things. You can connect identical batteries in parallel. Or you can use a bigger battery, e.g., replace a AAA battery with a AA battery (both 1.5 volts for alkaline), or replace a C battery with a D battery, in all cases space permitting. The same applies if you need more stored energy (power x time), but not more voltage.
Finally, the reason you don't see too many batteries in parallel is because unless the voltages are perfectly matched, one battery may attempt to charge the other causing damage in the case of a non rechargeable battery.. Instead, you use a bigger single battery (e.g., replace a AAA with a AA).
Hope this helps.