When your tomato transplants get really tall waiting for you to get them into the garden, they aren't very sturdy and less likely to hold the weight of the fruit. You can try digging a very deep hole to transplant them to keep those stems upright, sure. But, the better way is to bury them in a trench.
Laying the plant sideways allows all but the top few inches of the plant to be buried underground. New roots will form all along that now buried stem, giving you a very healthy root system to support that growing plant and the subsequent fruit.