FIDELITY IS A BY-PRODUCT. “The structure of the standard genetic code is nonrandom and ensures high robustness of the code to mutational and translational errors. However, this error minimization is most likely a by-product of the primordial code expansion driven by the diversification of the repertoire of protein amino acids, rather than a direct result of selection.”
http://www.annualreviews.org/doi/abs/10.1146/annurev-genet-120116-024713