Making a bunch of creatures at instant speed for a simple tap effect is quite powerful. Rakka Mar just enjoys having extra creatures. But, Omnath has the ability that when an Elemental token dies, you get to deal 3 damage to a target creature or player. Yes, the tokens have to be removed at end of turn, so they won't necessarily trigger that effect. Still, that's essentially free damage every single turn. Also, if you do have some way to sacrifice them, there's your free damage. Omnath, Locus of Rage has become an extremely popular Commander since his release, so it wasn't surprising to see this card being bought up suddenly when his popuylarity grew.
With Purphoros, his ability deals 2 damage to each opponent for each creature you have come into play. Enchanting Purphoros with Elemental Mastery and tapping him makes at least 6 tokens. That's 12 damage. This synergy is pretty obvious. That's a huge amount of damage dealing with only 4 mana.
Being such a useful card in Elemental Tribal Commander decks, plus Thromok the Insatiable decks, Elemental Mastery is a card that's often tricky to find in near-mint condition or better. It's affordable, and lacking a reprint, is a great investment for your Magic collection.