The computer controls your idle by adjusting the ratio of fuel and air based on a large numbers of parameters, i.e. engine load, temperature, altitude, etc.
Having a vacuum leak screws up the whole system because it introduces "false air". Note the following thread and the comments by Stevebfl:
Reving 300 E at idle?
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".