The truth of this is affirmed by the Tzimtzum itself. For the Tzimtzum took place only for the sake of the creation. If it had some other purpose, it would have had a different outcome. Since we see no other outcome of the Tzimtzum except the creation, which is its true outcome, we may say that the Tzimtzum was for the sake of the creation.
Further, His act of Tzimtzum prepared the way for the creation to come into being in a way suited to the nature of the created realms and beings, which exist within limits. If so, the Tzimtzum was for the sake of the creation. What is accomplished by all of Eyn Sof’s other powers – with the exception of the particular power that is the cause of the creation – is not for the sake of the creation. If so, the Tzimtzum took place only in that which is for the sake of the creation, namely in His power to bring about the creation as a work outside of Himself.
In other words, among His limitless powers there is one power – the law that goodness bestows goodness – which is the power to create realms and beings that exist as separate entities “outside” of Him. This is the power that is affected by the Tzimtzum, for initially this power was limitless, but He contracted it in order to create beings that exist within limits.