It shuts down other effects as well - your opponents can't cast spells from anywhere other than their hands, so no casting spells using Madness, Plot, Mayhem, or a whole bunch of red impulse draw effects
While some will tune their deck with expensive fetchlands and Doubling SeasonDoubling Seasons, Omnath can also just play a single Explosive VegetationExplosive Vegetation and smash in to cut out a fourth of someone's life total
Creature Artifact - Construct Phyrexian Color tag templates(None) Sea-Dasher Octopus Mutate (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own
If you like to get into the nitty-gritty about what makes a deck tick, or you just like reading primer-style articles about cool commanders, check out my series
Take a look at MaRos articles describing new sets - he does not just dump the image gallery and say let me know what you think - he describes the sets design goals, the mechanics, why they were chosen, how those mechanics were placed in each colour, the limited archetypes the set was designed around, some anecdotes about the design process, etc