Why do most smart light switches need a neutral wire?
Put simply; the smart switch contains electronics that needs power to operate, while a regular switch just mechanically cuts the load.
In order so that the switch can be 'on' to receive the RF signals, it needs a return path for itself so the 'load' isn't on all the time. Having the neutral wire present at the switch allows this to happen, and then when you command the load to turn on, it can pass through all the electricity to the load and thence back via that neutral wire to your load center.
What more important; what if you don't have a neutral and use a 2-wire module...
In those cases the smartswitch will be added to your electrical system in serial, instead of parallel. Because the smartswitch needs to be powered to be able to receive commands from the homecontroller, it cannot completely switch off, so it will always "leak" a certain amount of power. This power is not enough to be noticed on a regular light bulb (though on led lights it might show, as not being able to turn the lights completely off)
This is also the reason you see those devices (2-wire devices) only as dimmer modules, not as switches.