There were a few reasons that the elders told me,
raised beds give added protection to the plants/vegetables from the weather.
You can control what happens in the raised bed, so if you have a disease then you can control it to that part of the garden without it spreading to other plants.
and you can control pests, depending on how high it is you could possibly deter mice rats and other 4 legged animals.

I like them because you can determine a bed from a path, lol, when I first started growing it went straight into the ground, and my kids ran all over the plants I had in. not good. lol