As a people we generally enjoy having a home. A home is were we can be comfortable and safe. It is were we enjoy many of the things we require in life such as food and water. No different than us, plants need a comfortable space to acquire food and water.  This comfortable space for them is the soil. 
Soil is a conduit for many of the things that plants require. It supplies anchorage, water, oxygen and nutrients. 
Probably the most obvious purpose of soil and yet one of the most important, soil is a place for a plant to hold itself up. With the adequate amount of soil and space a plant can freely grow deep, strong roots. These roots will determine just how healthy and strong a plant will be. In hydroponic settings where soil is not used there is an artificial support that is used such as a netting. 
All living things, with the exception of some micro organisms, need oxygen. Plants do release oxygen during photosynthesis  but they also use it during other plant functions such as respiration. The oxygen in the soil is used by the plant roots as well as small soil organisms that promote plant growth. This is why it is important to have adequate soil aeration, having enough oxygen in the soil. Things that help to have good soil aeration are proper soil tillage, avoiding over saturating soil with water, and incorporating mulch mixtures with the soil when possible.    
Roots are responsible for water absorption for a plant. Since roots are mostly underground this is where soil comes in. Think of good healthy soil as a sponge. It absorbs all of the water that comes into contact with it. As plant roots grow they reach this stored water which can then be used by the plant. When watering your plants keep in mind that as a rule of thumb, every pound of dry matter produced by plant growth requires between 24 and 120 gallons of water.
While plants are able to absorb some nutrients through leaves, most nutrients are taken into the plant through the roots. Plant roots do this by absorbing nutrients that are broken down in the soil by both water, natural occurring minerals such as fulvic acids, and micro organisms found in healthy soils. This active movement of dissolved nutrients into the plant cells is called a soil solution.