get_output_key method is commonly used when working with txs and their key images. Because the method is not const, passing blockchain object though const& or pointers to const is not possible in this context. This is especially problematic in external projects (e.g., projects in moneroexamples) that use monero C++ api to operate on the blockchain and txs.
Thus, having get_output_key method will simplify moving blockchain object around through const references and pointers to const objects.
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