In the Arena, your opponents are human and your rewards can be significant. When you enter the Arena, after paying the 150G entry fee, you get to keep battling until you secure nine wins or three losses, whichever comes first. If you lose three times in a row on your first attempt, you'll leave with an Expert Card Pack and 40 Arcane Dust. If you win some rounds, your prize will be even better. For that reason, it's a good idea to get some practice before you enter the Arena, so that you can have a better time battling once you do enter and hopefully come away with better loot.