It is used to buy things from players on the market

Additional Market slots for trading, and a consistent supply of Rare or Legendary Crests to help power Elder Rifts. This isn't a bad bargain and the additional room for inventory is nice however, from our experience the Boon of Plenty doesn't bring any value aside from the indispensable Legendary Crests it gives.

You can purchase Eternal Orbs directly through the in-game shop. However, this is generally not a good deal , and it's probably the most inefficient use to spend your cash. Eternal Orbs allow you to buy crafting materials , as well as rare and legendary Crests, and can also be used to purchase cosmetic bundles as well as to purchase Platinum.

It is used to buy things from players on the market. If you're willing to keep pumping money into Diablo Immortal to gain more advantage in the game, then buying Eternal Orbs is how you do it, either by making use of them to purchase Legendary Crests or using them to purchase Platinum and buy Legendary Gems or upgrades directly from other players.

Playing Diablo Immortal without spending money is certainly possible cheap d2r items while enjoying the main campaign as you would a single-player ARPG is an enjoyable game that feels like there should be some amount of cost to it. If you plan on sticking with Diablo Immortal for the long period and take on the most challenging challenges at the end of the game, then you're an open-to-play player.