Except the parent post explicitly only cares about the type of key, which you don't need to parse the ASN.1 to know.
> a key use tag? Then each algorithm standard specifies it's own format for a blob of bytes that developers not doing serialization don't have to worry about?
As a developer passing blobs around, you already don't need to care that they're ASN.1, but you can care that you see a private key sitting around.
PS: and more than just X.509 uses those style of headers, too...