The blow-off valve prevents backpressure pulses from damaging the turbocharger and related plumbing when you chop the throttle (off) while spooled up.
If you stay off boost you'll be OK, if not you risk a harmful situation.
RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8