What I mean is that if you need to parse a format that has variable-sized structures, you can't just memcpy them into a struct because structs have a fixed size

What I mean is that if you need to parse a format that has variable-sized structures, you can't just memcpy them into a struct because structs have a fixed size. Additionally sometimes you need to parse a value into a more tractable representation, for example those octal ASCII strings in the tar format, which requires some additional processing that isn't possible with loading/serializing a struct.