Corkies first role is used as a drift bobber. It keeps your bait floating in the strike zone. With corkies now, they come in all different sizes, shapes and colors. I personally do not like corkies. If they ride up and down your line they can weaken it. When a fish bites, the corkie is hard and unnatural. The fish will spit that sucker out.
I only use puffballs now, or fishpills. Same thing. I like the Mad river fish pills. They are a soft foam ball that come in different colors. You run the hook point right through the middle of them and they keep your bait off the bottom, riding perfectly in the strike zone. They also keep your hook from snagging bottom and saves a lot of gear from breaking off.