Whether your tomato plants are transplanted or growing from the ground, they are probably stretching to the sky. Tomatoes need support as they grow. Trellises help keep plants upright, protect stems from breaking during turbulent weather, increase air circulation for plants and also help ensure that adequate sunlight reaches all parts of the plant. There are many options for trellising tomatoes, some of which are explained in this video.
There are many DIY options for trellising as well. Cucumbers, melons, squash, beans, and hops also benefit from trellises.