you use nujol when you have a solid and combining the solid with the nujol forms a toothpaste consistency which you can then perform a neat IR on.

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to provide a protective layer or coating for the sample if the sample is a reactive sample. forming a protective layer or coating would prevent decomposition of the sample during the IR reading process.

Raman is used a lot as it is not sensitive to atmospheric water and CO2 usually won't stand out on the spectra. Its also useful in most settings as there is no sample prep needed, which is quite a difference to somthing like IR spectra which need nujol mulls or KBr plates. In comparison to IR the bands of the spectra are usually smaller and sampling is non-destructive. In an industrial setting raman can be used with fiber optic cables to remotely monitor reactions and product formation.