You could plant your tomato seedlings and then just let them grow as they please, sprawling on the ground, but that's not a good idea. When touching the ground, tomatoes tend to rot because they are almost always moist on the bottom. When on the ground, bugs can reach them very easily. For those reasons and more, most people grow tomato plants upright, supported either by stakes or cages.
I choose to support my plants with cages because they afford more protection to the plant than stakes do, the stems are touched a lot less, and they seem better supported in cages than by stakes. I've also read that tomatoes produce better when caged. I use mostly 54 inch cages.
Since the cages tend to wobble from side to side, I use one stake per cage to steady it. I sink a stake at least a foot into the ground by hitting it with a rubber mallet, then I tie the cage to the stake with a 6" plastic cable tie.
When tied to the stakes, the cages are much more stable and will support the tomatoes better.
If you choose to stake your tomatoes rather than cage them, just remember to tie the stems to the stake a little loosely and with material that will not dig into the stem of the plant. String is not a good idea; it will cut into the stem. A lot of people use strips of old pantyhose because they stretch and "give" a little. They are also gentle on the stem. Some people, including me, use a stretchy green plastic tape sold in garden centers.