Defeat a Boss at +1 Difficulty
Once you've allocated your points, you'll face the Boss at +1 Difficulty. Defeating the Boss at this level will earn you +2 Atlas Points again.
Repeat this process
Continue allocating points and defeating increasingly difficult Bosses until you unlock Difficulty 4, the highest difficulty available for that Boss.
Bosses and their unique gems
Once you've unlocked Difficulty 4, you can now challenge the toughest Bosses in Path of Exile 2 to earn unique gems. These bosses will rarely drop gems, and Path of exile currency while you can't guarantee it, there's a chance you'll get one with every attempt.
The gems dropped by these top bosses often provide huge bonuses to specific attributes or offer unique builds and synergies. Even if you don't plan to use a specific gem for your build, the high value of these items makes them great to Path of exile currency for sale sell on the official marketplace, often yielding Sacred Orbs or other forms of currency.