Generally the flaps in intake manifolds are there to improve emissions and round out the torque characteristics of a given engine. I'm not familiar with the NA 606 so I can't speak on it's behalf. I do know that it has a sort of "throttle" valve in the induction pipe. That flap's purpose is largely to help with the EGR system at lower RPM's. Some of the earlier NA 240 and 300D's had a similar system for similar reasons.

If there are flaps in the intake runners, they are generally there to improve swirl at lower RPM's. By adding a restriction in the intake stream you get a higher velocity intake charge which helps combustion and cylinder scavenging. The difference is slight, but it is measurable - usually with fuel economy. Seat-of-pants tests usually won't yield anything appreciable.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)