Well, it says that if you spend your mana on an X spell, you get to draw a card and gain life equal to half of X, rounded down
Then these surveil 1 on arrival to the battlefield, so they can give you card draw security, and then also send it to the graveyard if you don't immediately need it, or just want reanimation, flashback or delirium for later use
4x Swamp
This is especially true if you make plays according to your opponent's exact preparations
In many games, you will not cast your Commander at all
Just so long as your deck has a high enough density of creatures, youll barely feel the impact of Thalia, while she frustrates all of your opponents and delays their plans