The most useful function on my laptop is the power-saving function for when I'm running it off batteries. It automatically goes into dimming mode, slows the CPU down a little and switches off the screen after about 2 minutes inactivity.
The most useful function of my laptop is as a notebook. When I'm sitting on the train and have nothing else to do, I can draft essays, letters or whatever.
Functions on your computer are those things built in which help you perform the tasks you need to perform. Functions of your computer are the types of work you can put your computer to.