I've written before about the mastery system in Magic Arena. TLDR 4 wins a day will comfortably max out your mastery.
The mastery upgrade costs 2200 gems and at level 100 gives:
4000 gold (worth 600 gems)
1 ranked draft entry (worth 750 gems)
Booster packs: 4 GRN, 4 RNA, 4 M20, 4 WAR, 4 ELD Key: here.
Random cards: 8 mythic, 10 uncommon
A large amount of cosmetic upgrades including the owl pet.
The first 3 items alone clearly return more than the gem value making this well worth it. It's effectively 1150 free gems if you're an active player.
The other benefits are nice to have on top but not benefits I'd pay money for. But unless you're committed to playing for completely free (which is perfectly viable) then this is an essential upgrade for anyone who is willing to spend money on this game.