A lot of people have been shouting that poise does nothing, but they’re idiots. It’s just different. A decent amount of poise gives you a substantial advantage if you’re wielding a heavy-ish weapon against a light weapon. Fill up your equip load allowance with as much armour as your 15 vit allows, then you can swing a two-handed greatsword against a sword, spear, axe or scythe and poise right through their attack. Works great against overly aggressive players who like to spam R1 every chance they get.
Also, the heavier your weapon, the less poise you need. With a greataxe, curved/ultra greatsword or great hammer, you can poise through light weapons even with a small amount of poise. If you actually want to wear medium armour with an ultra weapon, you’ll probably need to boost your vit to at least 20. If you do, you’ll just be able to trade with greatswords and win.
If you go full heavy, towards 40 vit and higher, and use the heaviest weapon and heaviest armour available, you’ll be able to win all trades no matter what you’re up against. It takes a lot of stat investment, you’ll need plenty of endurance to swing that heavy weapon around, and plenty of health so you don’t just die when you tank a hit from an ultra weapon. Fighting against light weapon wielders will be difficult, but you’ll be unstoppable in heavy vs heavy.
How much vit you’ll want depends on how you want to play. With the Lucerne, you’ll be able to get devastating counter hits with the Leo ring if you trade hits with people. But halberds have very weak poise, and it only applies when two-handed. You’ll need to armour up significantly to compensate. Even poising through light weapons consistently requires heavy armour - 38 poise is the breakpoint for that. But if you ask me, it’s worth it. A poise playstyle is immensely satisfying.
For more info, you should watch this video explaining poise.