I think its best use is harvesting berries. It is basically the midgame trike, although i like the trike more with combat. The stego has a slow attack, but is has a large hitrange, and so it harvests lots of berries at once. It also has a decent carrying capacity. I suggest to upgrade weight and melee.
Once you manage to sit through the tame, these things can be absolutely amazing for gathering berries.
You'll need two people, a quetzal, and your shiny new stego.
Have one person on the quetz, one on the stego, and have the quetzal pick up the stego in its big chicken feet.
The quetz may now fly around, very close to the ground, and the stego can swing to get constant berries.
Don't spam the "swing" button, or you won't gather anything. Wait until the swing animation is over, and then tap it again.
When the quetzal is encumbered, they can drop the stego to regain stamina, the person on the stego can drop the undesired goodies, and you can go off again.
Stegos are the new turtles. Worth finding a high lvl M & F. Need a few sarco (or 50) for kibble, but then pow! You've got yourself a bullet sponge that protects the rider from turrets. Magnifine glass will be your best friend to find the right ones to breed.
Used four of these with ten Rex’s, a yutyrannus, and three megatheriums for the beta broodmother fight. Went very well as the steggos can knock back waves of araneos at a time, though I do suggest leveling up hit points and melee high before you do.
Stegosauruses are very good at collecting berries, many people think they are better at collecting berries than Triceratops. Even though they are slow, they can defend you with their powerful knockback. These are a good source of prime meat, because they are one of the easiest dinosaurs to kill which gives prime meat.
A stegosaurus can collect hundreds of berries in a few short seconds. You can use those berries to tame more dinosaurs and feed yourself very easily. The only problem is that they are very slow, so if a large predator attacks you while your on your stegosaurus, it would be better to jump off and run away while your stegosaurus distracts it. You may lose your stegosaurus though