Well, I've wondered that myself many times. I also thought that I needed to replace it because the felt arround it was all shot. I don't think this is the case. The part looks like a check valve, but it's not. I think it's primary function may be to stabalize the vaccume in that line which leads to the "Pressure Sensor" this is the part mounted on the passenger strut tower. The funny thing about the pressure sensor, at least according to my Haynes manual, is that my car, as well as yours (looking at your profile "5spd") does not need a pressure sensor. Well, surely it needs it if they put it on there? Well here's what the Haynes manual has to say about the pressure sensor:
"The pressure sensor monitors the intake manifold pressure changes in engine load and speed and converts the information into a voltage output. The pressure sensor is used on turbocharged and 1995 and later automatic transaxle models only..."
hmmm... So why does my 5spd have one? It's not turbo (yet, hehe..) and it's not automatic. My thoughts are, the computer still uses it to help calculate load and adjust fuel and timing accordingly. Maybe it's not conventional to use that sensor on a 5spd, non-turbo car and the Haynes writers wrote that little blurb, not knowing any better, since their car was almost certainly an automatic outback. Just my guess. :shrug: I may have to look into this some more in the factory service manuals.