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.
With the new TLC the stego can easily become a must-tame early game dino for many players. It has 3 stances when ridden you can change between pressing C (crouch):
Heavy plate, boosting thatch gather rates and applying a heavy slow on hit targets.
Sharp plate, boosting berry gathering and attacks penetrate armour (useful for PvP).
Hardened plate, boosting wood gather and halves damage taken.
For combat, heavy plate is great for effectively rooting enemy and wild dinos in place, whilst hardened plate is to tank everything that comes at you. Sharpened plate shines together with allies that can slow and/or tank whilst you focus on dealing good dmg.
The alternate attack of the stego can impale creatures up to the size of roughly a parasaur, dealing damage based on attack damage. In order to do so the target must have a certain amount of health left, which is highlighted with the crosshair turning red when looking at the target (unsure how it is shown without crosshairs).
I have a stegosaurus called Strega and she’s a GOD, can kill five giant beavers in a few swings, has very high health, doses decent damage, and helped me and my only tribe mate, my dad to tame our first giant beaver.
Very useful in PvP to soak turrets because if it’s abilty to protect the rider and high HP. Back into turrets as it’s tail takes reduced damage, beware of tek turrets as the splash dmg can still get you. Also it’s helpful to bring a few sweet veggie cakes so the stegosaurus can regain some health. (Up if you found this helpful)