Air can be forced out because of too much water. Water is also a necessary ingredient for soil. Water is necessary for plant growth. Plants need it just like we do. Water also carries nutrients through the soil.
Organic matter is a requisite for healthy garden soil.
It holds water, stabilizes the soil, holds nutrients and provides food for soil organisms. All the microorganisms in your soil help your plants. Don't be afraid of insects, bacteria or fungi in your soil. The vast majority are benign or helpful to your garden. The fungi and bacteria in the soil help feed your plants, by breaking down dead organisms and creating molecule your plants can use.