This is because Garth can be tapped in order to allow its controller to cast a copy of one of six spells: Disenchant, Braingeyser, Terror, Shivan Dragon, Regrowth, and even the famed Black Lotus
You also get some information early on about what kind of deck your opponent is playing with Merfolk Spys triggered ability
But I wanted to get one stock game in
Simply flash a bunch of creatures in prior to your turn, play spells as necessary, hit an opponent with your creatures, untap your lands, and you essentially have free mana to do the same thing again
You might also want to include as many orcs and goblins as you can for sacrifice fodder
Lightening Helix 3.0 Power 3.6 Support 2.4 The helix is vastly overrated by most players in most formats