Tzeentch is the god of Change, not simply Knowledge. Secrets, Mutation, Scheming, and Sorcery are all part of his purview. He thrives when everything is as complicated, contradictory, and random as it can possibly be, the very literal endpoint of utter chaos. Generally, he's depicted as unfathomable because of all the plots, schemes, and plans he has running simultaneously, many of which conflict or outright oppose/counteract each other, because it's not the victory that empowers him so much as the act of scheming in the first place - and because his plots are so convoluted, it requires equally complex counter-plots to defeat them, which also gives him strength.
There's a story from one of the old rulebooks that I think embodies Tzeentchian attitudes best. Imagine a factory worker in a hive, whose job is to pull a lever every time a bell rings, for his entire work shift. He doesn't know what the lever does, but his job is to pull it. Eventually, he starts wondering what the lever does, but can't find out. He's bored, wanting even a tiny bit of variety in his life, wanting something, anything, to change. So one day, he waits thirty seconds after the bell to pull his lever.
Now, that lever is part of a mechanism delivering fuel from a promethium refinery in the depths of the hive all the way up to the shuttleports at the top. The lever controls a valve that opens and closes to move that fuel around the pipe network, and needs to be regularly opened to lower pressure. When our simple worker delays his lever-pull, the pressure builds up just a bit too high, and part of the pipe network explodes. The explosion damages the secure cells of the local Arbites precinct, releasing dozens of violent criminals and heretics into the streets. Riots erupt in the slums, and forces are called in to contain them - but due to a lack of fuel because of a broken pipe, the vehicles needed to reinforce the slums cannot move, and the slums are overrun. The riots spread, and within months the entire hive is in total rebellion. Unable to quell the disorder, Imperial Navy warships are ordered to slag the hive from orbit and kill all two billion inhabitants before the rot can spread. All because a bored worker sought change in his humdrum existence by being slow to pull a lever.
Just As Planned.
(Tzeentch's catchphrase/memetic slogan, for reference.)
2. Sisters aren't usually deployed offensively against Xenos-style threats. Defensively, though, they're frequently called on by the local Imperial authorities to defend Imperial worlds from whatever is attacking them, which can often include aliens.