First, lets establish what a Blow Off Valve (BOV) is:
The blow-off valve releases the pressure from the turbocharger before the inlet/intake manifold, when the throttle is closed the air has nowhere to go. It stops the air surging and going back through the compressor wheel. It's designed as a safety feature as far as the turbo is concerned.
Next, how does it work:
When the throttle is closed, a vacuum forms in the manifold. This in combination with the pressurized air from the turbocharger moves the piston in the valve up, releasing the pressure into the inlet of the turbo (Recirc.) or the atmosphere (Vent to atmosphere VTA).
Now lets get into the myths about BOV, these are not to live by but give general concepts and of course there are exceptions to every example:
Blow-off valve myth #1: One of the biggest misconceptions about blow-off valves is that you need to tighten the spring to run higher boost. This is totally incorrect (at least, for a GFB valve anyway), as you can see from the last paragraph, at full throttle, the pressure on the top and bottom of the valve is equal, therefore cancelling itself out. So no matter what boost pressure you run, the valve will stay shut.
HOWEVER : Having said that, some factory valves (including the one used on the edge) have systems designed into them to crack open at high boost, in order to protect the engine from boost spikes. They do this by designing the pressure to be unequal on the top an bottom of the valve, thereby overcoming the spring at a certain pressure.
Blow-off valve myth #2: The fluttering sound is usually believed to be the blow-off valve. In reality, it is caused by a blow-off valve, but does not come from the blow-off valve. If the spring pre-load is adjusted too tight, this will cause compressor surge, which as described above is the sound of air exiting the turbo.
Having established the basics, why do you need a BOV for your car?
If you intent is to tune and call it good, you do not need one.
If your goal is to go fully bolted (intake, boost tunes, down pipes, intercooler, and exhaust. you still TECHNICALLY do not need one however the stock BOV will crack open at your newly found higher boost.
If your goal includes the above plus better turbos you absolutely need one for precise control.
The control part comes from the factory springs and diaphragms typically having some "give", creating performance variance. So by using the TS or other BOV, you are tightening tolerances, and hopefully & concurrently, improving performance. On the Ecoboost engines, Ford uses a lot of parts that are under-spec'd from a tuner perspective, so you have hose clamps coming loose, hoses cracking, causing all kinds of boost leaks, etc. Fortunately the intercoolers have withstood the test of time LOL.
But yes, for most people, it will only be a "bling" mod. For those who tune their Edges for performance, it will allow the tuner to have more control over the tune. If the mechanical parts are "loose" in their behavior, you have to dial back the tune for safety.
So yes you are 98% correct. most people want the noise but it is also for the use of control as you stated.
Thnx Manu for chiming in, you're right on the money...
In addition, the stock BOV's spring is weak which allows for premature pressure/charge release.
Also as we Mod we need the ability to make blow off adjustments.
Special K is much more savvy in the specifics of this benefit, I'll ask him to chime in...
4 the record...we're not bling kinda guys.
We're form, fit & function logistics engineers.
We post to benefit forums members, not to boast...
hope this helps