Especially in towns soil may be so compacted, so nutritionally poor, so full of glass and pollutants... that raised beds may be the only way to garden. Here are some advantages of raised beds over regular gardening styles:
Studies indicate that raised garden beds may produce 1.4 to 2 times as much vegetables and flowers per square foot as ordinary garden plots!
Below you can see what it looks like while constructing beds:
Notice the black plastic sheet beneath the dirt. That's a special kind of sheeting perforated with tiny holes so that water will run through it. The plastic helps keep critters out from below, and tree and grass roots. I'd worry to put down other kinds of perforated sheet plastic because you never know what chemicals regular plastic sheet might disintegrate into.
Also in the background maybe you can see a wire fence to keep out animals. In many places deer, rabbits and the like will eat your plants long before the gardens produce a crop. If you're not sure whether you'll have animal problems in your garden, you need to talk to neighbors and maybe put up a fence. If you have deer problems, the fence will need to be very tall indeed.
Finally, the soil you fill your raised beds with must be good for gardening. Below you can see some garden soil that just by looking at it you can tell that it's good:
Good gardening soil is loose and crumbly. It'll have visible tiny pieces of decaying plant material (organic matter) all through it. It'll be dark. If it gets wet, water will soak through it, but then the soil will remain moist, not wet, for a long time. Finally, it'll smell rich and musty. You'll know good-smelling garden soil when you smell it.