Many cyclists always sit in one spot. Winter rides are a great time to experiment with moving around. Scooting helps you increase pedaling cadence or power.
For normal cruising, you should sit back far enough for your sit bones to be resting atop the saddle. This gives support while minimizing pressure on the nerves and blood vessels that run through the soft tissue of your crotch.
When you want to increase your cadence for brief periods, scoot forward. Try it and you'll see now much easier and more natural it feels to pedal at faster rpm.
When you want to increase your power, scoot back. This slightly lengthens your leg extension and puts you in better position to drive the pedal over the top of each stroke. This more-powerful thrust is especially helpful when climbing at slower cadences or turning a big gear.
As a bonus, scooting increases comfort by changing saddle pressure points and upper-body position. Get into the habit!