This highly fragile looking cable is in fact the famous Dreamcast Coders Cable that some home brew Dreamcast programmers and hobbyists use.  The cable basically provides a data transfer link between a PC and a Dreamcast so that data can be sent to and from each machine.  This is great if you just want to run a bit of code on the Dreamcast for testing since there's no need to waste a CDR.  The transfer rate can be quite slow though.  While it's perfectly fine for small chunks of code it can be quite a wait for bigger files.  So those lucky enough to have one use the Dreamcast Broad Band Adaptor which is much more faster.