I play it for 4 reasons:
- I like there's a little more at stake, and the opportunity to reap big rewards if you do well. I still play constructed occasionally for casual matches/daily challenges.
- I enjoy the drafting process, as cruel it can sometimes be.
- As a new player who hasn't spend any money on the game, I sometimes feel very underpowered vs decks that run various top-tier collectible cards, or even legendaries. In arena, I'm not affected by my small card pool.
- The reward packs help me grow my card pool for when I do play constructed. So far I've been good enough at arena to make it more efficient than straight buying card packs, though I do wish I could get classic packs as a reward instead.