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There are many possible reasons and trying to simplify the purchase of the Su-35s to only a single reason (such as "TVC") is not very conducive to good understanding.
We should view this purchase not only in terms of the capability the Su-35s bring -- because their capabilities are somewhat limited, given the price of buying them, especially in context of the current state of Chinese fighter aircraft (not to mention new fighters that will very soon enter frontline service such as J-10B/C and J-16) and their force multipliers such as AEW&C and EW/ECM planes.
Other reasons include demonstrating its willingness to cooperate with Russia on defence matters in a greater way, getting access to some recent Russian technology just to see what they've been up to (possibly including TVC, but also to have a look at other technologies such as radar, materials, etc), and also simply getting additional advanced Flankers to supplement SAC Flanker production.
In other words, there are probably a whole host of reasons which must be taken together to rationalize the Su-35 purchase, and they include geopolitical reasons as well as the capabilities that Su-35 bring, as well as possible interest in looking at certain Su-35 technologies as well. No single reason is likely to explain the purchase alone.
If this purchase of Su-35s occurred in the early 2000s then China could have gained a lot more from 24 Su-35s both in terms of capability and helping their own technological advancement, but now the benefits a mere 24 Su-35s can bring to the Chinese military aviation force and the industry in terms of technological advancement is far, far smaller.
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