The spreader bar is placed between the front of the front wheels, to "toe them out" if you will. The main purpose is to take the play out of the steering linkage. M.B.DOC says you don't need it if you have new parts. However, that has not been my experience. 55-75 ft-lbs (can't remember which) pressure it put on the bar.
MB spec for front toe (total) with spreader bar is 20 min or 0.33 deg (at least for my '87 W124). The first indy THOUGHT he set it to 0.33 deg, but I knew from symptoms that it was not. A second indy took the same reading. BUT the dealer read it as 8 min, and corrected it to 20 min ~ 0.33 deg. So, in my case, the dealer alignment resulted in a toe-in change of 0.20 deg & the car does better.
Note with the non-dealer alignment my tires were wearing perfectly.
If your tires are wearing on the outside (as all mine were when I purchased the car) then you either have too MUCH toe-in all over the place (which would not cause wind sensitivity & oversteer, but rather the opposite) or you have suspension components worn.
With the dealer alignment my tires still wear perfectly.