The hose is there to reburn crankcase gases in order to lower emissions. I saw a post on SH about this topic, and there's an explanation about how it interupts the whole intake system, and can reduce engine life. Pretty detailed but I started falling asleep reading it so I can't give a short, cliff note explanation. If you want the entire thing do a search on SH, or PM blackduececoupe on SH, and he'll tell you his version.