Loosen up the soil between roots
There are a lot of people that wonder why there's a tractor attachment that loosens up the soil. It's called an aerator. When you aerate your lawn, you are opening up the soil surface to let the grass roots get the air and moisture it needs for healthy growth. It also helps when you fertilize by letting the nutrients in the fertilizer easily reach the roots.
When do you aerate? The best time to aerate is in the spring at the start of the growing season. Other times, late in the summer and fall, are beneficial times to keeping your lawn healthy.