My yard is on an incline, so I have to have several beds to do my gardening.
The reasons for this amount to one word - drainage. This applys to not only water but air as well.
The lower end of my yard is in a shallow valley. This causes issues in the spring when the snow melt begins and my yard floods for days on end. In a dry land farming area, my neighbors don't empathize with me at all, but it makes it difficult to get down there to do any planting. So I have beds at the upper end of my house along the driveway, as mentioned in my first post, and on the south side of my house for spring planting.
This shallow valley also suffers from cold air drainage as well. There can be a 3 degree temperature change from that part of my yard to my mom's who lives 1/2 a block up the hill from me. This makes for a shorter growing season in that area of my yard. I often have to wait until the first week of June to plant this area due to temperature and drainage.
So the bed on the southern side of my house and the one on the east side of my yard, they are both uphill and are not affected by the drainage. They work well for earlier plantings, such as peas, or spinach, cool weather plants that don't need to be wet. The bed in the lower end of my yard is actually good for plants that require more water, such as corn, and by planting according to the these needs, I use less water.
So location is a large factor when planning where to put your garden(s).