The MD5 algorithm is intended for digital signature applications, where a large file must be "compressed" in a secure manner before being encrypted with a private (secret) key under a public-key cryptosystem such as RSA.
MD5 does the compression. RSA does the encryption. Taken together, this operation is called a digital signature. RFC1321 does not lay this out for you in full detail, because it is not meant to be "the source" of information about anything except how to implement the MD5 algorithm.

I'm not sure, at this point, what your beef is. MD5 was designed to have a certain set of properties. People used it, assuming that it did in fact have those properties. That assumption was shown to be false. Yet you seem (in a number of threads on perlmonks) to be arguing that people should continue to use MD5 anyway.

In reply to Re^11: On showing the weakness in the MD5 digest function and getting bitten by scalar context by Anonymous Monk
in thread On showing the weakness in the MD5 digest function and getting bitten by scalar context by grinder

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