What is the function of a crankcase sensor in a 2005 Audi A4 1.8T Cabriolet do?

There's no such thing as a crankcase sensor as engine sensors are normally defined by their function rather than what they are mounted to. What you are probably referring to is the knock sensor which is usually mounted on the side of the engine block. The knock sensor monitors detonation/pinging/pre-ignition and is used by the engine management system to offset the ignition timing.