MATH & SCIENCE
What could be better than the ability to say (or do) "let there be light!" and there is? For a toddler, the light switch is a great opportunity for them to develop confidence.
FINE MOTOR SKILLS
It is very interesting to watch your child practice until they figure out how to use the light switch. You can almost hear the gears in their brains going a mile a minute. It's easy for adults, but for the young child, it is tricky to figure out just how the fingers work to move the switch up and down (or other ways, depending on the lightswitch you have). Resist the urge to do it for your child! As long as your child is not throwing a screaming fit in frustration, they are okay.....just encourage them sweetly, or even walk off and leave them to figure it out on their own.
The light switch is especially great for problem solving if it has two switches, perhaps one for an out door light, for example. Then your child will spend many long and satisfying minutes testing and re-testing their theory -- this switch makes that light come on or go off....but this other switch does not! Is that always true? How about this time? Or again, this time? If you leave your child in a chair near the switch, they can be happily entertained for quite some time.
Suggested books about lights and solving problems