The main idea behind was in situation where you have SpamAssassin installed on the mail server itself and there's no use to have another spam procesing engine.
For instance I have SA installed on my home mailserver and I have the office email account (no SA at office) and I would like to use my home SA to identify junk.
On my office account I have Report as Spam (Rspam) Plugin installed so I didn't touch learn/report stuff. Of course spamc is still required (it usualy comes already installed in most modern distros). If this makes sense I'll add port option, ssl, and username option.
These details are provided for information only. No information here is legal advice and should not be used as such.