Taylor series expansion of analytic functions on domain $D$

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Regarding the Taylor series expansion of an analytic function, analytic functions are defined on the largest disc in side domain $D$, and is derived using the Cauchy Integral formula. Is there any specific reason to bound that expansion within a disc? I mean to say, Cauchy's Integral formula, can in general be applied on any weird shapes inside the domain, why not use that? We just need to make sure, it stays inside the radius of convergence. Please someone clarify.