As far as I know that orifice determines the boost cut off point as it relates to the wastegate actuator. With the '88 there is additionally the boost control valve for the Reg./Premium fuel requirements in the vacuum/pressure circuit. Also, since the Automatic cars ran less boost that fitting in the intercooler has a different orifice size from the Manual trans cars. So, yes, it would be necessay for the stock system and the boost level is dependent the orifice size. Again, - as far as I know.
On my car the only thing that is on the vacuum tree is the Source (which is at the upper plenum and is shared with the fuel regulator) and the boost gage line. Basically I could eliminate the tree and just run the gage line to the plenum fitting. Now, I have eliminated the EGR, and though intended to someday..., have not hooked the cruise control up - yet - if ever. Going by what is called the Asperator in the diagram Jerry provided above I believe that is just for the cruise control. And that might just be the vacuum bleed valve on the brake petal that I assume is a fail safe to release the cruise control whenever the brakes are applied.
http://www.turboford.org/faq/boost.shtml Article on the specific boost control system