cars will normally have a boost limit to stop any major overboost, ie; split pipe etc, you can only normally raise the boost a bit before you hit the standard boost limiter. a bleed valve is a great bit of kit if treated with respect, there crude but they work, great on cars like RS Turbo's where if you were working on it on a dyno you would have to remove the car from the dyno, jack it up & wait for it to cool down before adjusting the boost on the actuator, with a bleed valve you could just turn the knob a small amount & try another run!
the big problem with them is, people mess with them, crank the boost up but then theres not enough fuel to have a correct mixture, this normally leads to det or high heat then pistons normally start to melt............