I've seen this quite often where the poster tries to reduce the file size by RARing source files, ZIPping the RAR then re-RARing the ZIP
It makes no difference to the size of the original RAR file but is a PITA. However, I agree with Hecks that this is quite often a technique used to hide a malicious payload as AV programs usually only check the first level of a compressed file (ZIP or RAR). Personally, I wouldn't want this as an default option as I like to be in control of situations like this.