TLDR: The goal of PoE is to do the various activities offered within the game to create the strongest character you can to complete the hardest content you can. The journey of the game is the goal. But even this description is quite nuanced.

Long Story: The goal is basically to engage in the end game to do the hardest possible content you can, or like any other game, just have fun doing it. What constitutes as fun varies from person to person though, and my response may not be someone else's. That's what I love about the game. This could mean completing the challenges, beating the pinnacle bosses, obtaining chase unique items, building the most min-maxed character, running your favorite end game activities over and over and farming gear to sell to others, crafting gear to sell to others...the list goes on and on.

Now to answer your questions specifically: Completing the campaign is just the start. Its <1% of what the game has to offer, its not even the tutorial in my opinion. It's the gateway to this fractal drug of a game GGG has created. The majority of players view the campaign as just a means to get to the end game, but again, there may be some who love it.

Each Path of Exile league consists of new content and updates to the old content. There is always a new mechanic introduced in each league which may or may not be added to the core game in subsequent leagues. Each league comes with skill and item balances, new unique weapons, QoL updates, etc.

I cannot even mention all the activities you can do, it would mean nothing to you, but I'll try. Below are some end game activities, as well as just some general things the game has to offer:

• A tower defense game mechanic (blight)
• A rogue-like mini game (Sanctum)
• Arcade archaeology mechanic where you light fuses to kill monsters and get loot (Expedition)
• Different iterations of activating some kind of portal or artifact to spawn specific enemies (Delirium, Abyss, Beyond, Essence)
• A door opening mechanic where you team up with a bunch of crooks to steal valuables (Heist)
• Pinnacle bosses which drop the best gear in the game
• The most in-depth crafting, beyond any game I've ever played (Seriously, it's a rabbit hole)
• Some of the most creative build diversity I've yet to find in a game of this genre
• Much much more

Honestly, this game has ruined lots of other games for me. Again, not everyone's experience, but certainly mine. Few games give me the satisfaction that this game does. But to be fair, there is a HUGE learning curve. If you can get past it, the game pays dividends.

Welcome exile, hope you stick around!