I have disabled NVidia Optimus on my Dell Precision M4600 since I cant dim my screen if it is enabled for some strange reason. So, what does that actually do? Does it use the nvidia card all the time then or what?
What does it do? It "intelligently" switches between the NVidia GPU and the Integrated GPU in a system. This is to save power. If you are surfing the web, reading your email, there is no need to be using the more powerful NVidia GPU. This would shorten the charge life of your battery, requiring you to recharge it sooner... if you were using it all the time for mundane tasks. Since the NVidia dedicated GPU also generates more heat, this causes your laptop to run hotter. Laptops in general have issues shedding heat in the best of circumstances, thus generating more heat when it isn't necessary is typically frowned upon... performance-wise.
Now... with Optimus running, when you play a game or use a program that requires more GPU, the dedicated NVidia GPU is turned on and you get all the joy and happiness associated with a powerful graphics processor when you need it.
Turning it off will typically disable the dedicated GPU altogether, saving battery power but making your game performance suck.