Charles Spurgeon Commentary Romans 4:22-23

"Wherefore also it was reckoned unto him for righteousness. Now it was not written for his sake alone, that it was reckoned unto him;" — Romans 4:22-23 (ASV)

And therefore it was imputed to him for righteousness. Now it was not written for his sake alone, that it was imputed to him

The imputation would be enough for Abraham without any writing; but as it is written, it is for our instruction, and for our comfort.