The 30mm wrench holds the valve spring retainer in place so that you can tighten the two locknuts against each other without the whole assembly spinning.

 

That's pretty much it. The first time I did mine I needed it on only one valve, the rest I didn't have a problem with. A lot of people just jamb a flat screw driver against the side of the springretainer to hold it. I just figured as long as I was making the two 14mm wrenchs I would make that one also.