Are you taking off from a grass or gravel runway? Is your hard-surfaced runway covered in snow? Then it's time for you to show off your soft-field takeoff skills.
How is a soft field takeoff different than a normal one? It all comes down to one goal: transferring your aircraft's weight from the wheels to the wings as quickly as possible.
Why? By lifting off as quickly as possible, you eliminate drag from grass, sand, mud snow, etc. And that's important, because excessive drag on a runway can dramatically increase your takeoff roll. Just look at this example: