If you shift to a bigger ring in front without shifting in back and keep the same cadence, ie. turn the pedals as fast as before, you will experience more effort and go faster. Handy whenever downhills, a tailwind (sorry, said headwind in the last post, that must have been confusing) or whatever makes you feel that you're expending too little effort and could expend more. It's like taking longer strides when you're running.

Vice versa if you shift to a smaller ring without shifting in back, and keep the same cadence, you will experience less effort and go slower. Good for uphills, a headwind or any time you feel that you need to go easy. It's like taking shorter strides when you're running.

Expending too much effort at a low cadence can be hard on your knees, expending too little at a high cadence can cause you to bounce around and not really get anywhere. You just have to try and see what feels comfortable. And yes, you can customize the rings to a certain extent, and get a larger largest ring if you feel you're constantly "running out of gears" downhill and have to coast a lot.