I can use this opportunity to explain the rationale behind RSN.
Solid archives
Often, SPC files will only differ by a few KB, with the rest bit-identical. For example, several songs might all use the same playback code and set of samples, with only the module data (or even just the playback pointer) differing between songs. But because PKZIP always compresses each file as a separate object, it can't take advantage of similarities between files. Even tar.gz, which combines the files before compressing them into a "solid archive", could only reference data from the past 32768 bytes. RAR was the first widely used general data compression program that used a window bigger than a single SPC file, so SPC sets were often distributed as RAR files.
Why specifically RAR
The one disadvantage of RAR is that it is not
free software due to a license term prohibiting reverse-engineering the format to create a description of the compression method. 7-Zip and tar.bz2 are free alternatives to RAR, but RAR had the first mover advantage, and music archives that already deal in non-free music have seen no need to repack in a free format.
Why the rename to RSN
The file manager included with Windows is incredibly primitive. Instead of inspecting each file when the user opens it to see what application is best suited for it, it guesses based on the part of the filename after the last period character. That's why various file formats based on PKZIP archives have different extensions: .zip for general data, .jar for Java applications, .smzip for StepMania, .cbz for sequences of JPEG or PNG images ("comic book zip"), .odt for OASIS text documents, .ods for OASIS spreadsheets, etc. Likewise, RAR has .rar for general data, .cbr for image sequences, and .rsn (RAR Super Nintendo) for sets of SPC files.
Feel free to copy this to any wiki you want, as long as I am credited. What is the most popular wiki for Super NES development info anyway?