If you get the sha256 from the same place you got the main file, then anyone tampering with the main file could also recalculate the sha256 to match the tampered file. A signature signed with a certificate uses complex math (public-key asymmetric cryptography) to give some certainty that the signed content (the sha256) is the same sha256 that the original file author created. It’s not mathematically feasible to recalculate the certificate signature. Why don’t we just sign the whole original file with the public-key crypto and skip the sha256? Because asymmetric crypto is much, much slower than plain symmetric crypto or hash functions. It’s faster and easier to generate the valid hash or key, then sign or encrypt just the smaller key.