It is used to control diaphrams on the HVAC system, It and its brother "pressure" is used in the lock and unlock process on some cars. It is definately required for the power brake assist. It is used to control shift points on transmissions, has a role in emission control devices. On old cars it was needed for the windshield wiper motor, and even the washer pump. There are probably other applications, head lamp doors, cruise control diaphrams, some of these MB has used and is starting to go away from.
A vacuum pump is required on diesels since the engine does not provide any vacuum at all to run all this stuff.