Higher vegetable yields:
You will get 2x's the vegetable production out of a raised bed than a standard flat earth bed of the same dimensions. You can plant vegetables closer together because of better soil conditions. The higher yields are possible because you are protecting the gardening area from compaction. If a vegetable plant can grow downward and outward without their roots being stomp on and compacted, the plant will thrive. Thriving plants produce more. Plants can be grown closer together because there is plenty of room for their roots to extend deeply downward. They don't compete for space.
You can add garden soil, organic matter and all the other good stuff to your raised bed in a focused and controlled manner. Soil compaction is greatly reduced, as mentioned above, because you never step in the bed. Soil compaction inhibits plant growth, oxygen uptake, and water circulation. Because you are framing off the specific growing area, you can better control what you put in it. It is much easier to add sand and organic matter to clay soil when you have a framed area. You can also purchase bulk soil delivery from your local nursery and fill the raised bed. You dig out the "bad soil" and dump in the new. For some areas it is easier to remove earth then amend it. Raised beds make this task much easier.
Your raised bed will drain more quickly and be less prone to staying soggy and thus promoting disease and seed rotting. Typically you should dig a raised bed down at least 2 feet. By doing this you create a garden bed that will drain water from the top on the soil to the bottom. This is a much healthier environment for your seeds and plants.
Better air circulation and more sunshine:
Raised beds tend to get better circulation and more sun depending how you set them up. Air circulation helps cut down on plant diseases. The standard raised bed plot should be 4 feet by 8 feet. You create the width of 4 feet because your arms have about a 2 foot reach. This allows you to tend to the entire garden from the surrounding walking paths. You don't need to step in it. By creating 18 inch to 2 foot walking paths between the raised beds, you create space for air circulation and more sunshine. Circulation and sunshine are paramount in disease prevention and strong growth.
Better water conservation:
You only water where the vegetables are growing. You can also install slow drip soaker hoses in the bottom of your raised bed garden. If you dig out 2 feet of soil you can place a slow drip soaker hose in the bottom of it. It should be put in - in a snaking pattern. It has to be deep enough your spade won't reach it. By focusing your watering habit directly to the vegetable growing area, you will save money and water. Depending on how much of an edge you put on your raised bed, you can also flood the top of it. Simply fill it up with water and then let it sink away. Because you amended the soil the water will use gravity to get deep into the bed. This helps prevent damage from soil that dries out to deeply during the hot summer months.
Easier weed control:
Weeding is easier and because you can plant more plants in a raised bed garden, they tend to shade out weeds. The soil is loose. It is much easier to "fluff" it and turn it and thus burry weeds. Pulling weeds is easier because as mention the soil is so loose. Dandelions come out with a full root! Mulching for weed control is less expensive because it is done in a targeted area. Mulching not only controls weeds, it helps conserve moisture. And mulch will add organic matter to your garden when it gets turned at the end of the season.