The audio Mastering process, is one of adapting a mixed recording to device media, since it isn't necessary to record "hot", as dynamics are faithfully reproduced, due to higher bitrates being used, however the noise floor of individual samples may be raised.
Applying equalisation and compression, to audio files allows them to be adapted to systems exhibiting limited frequency response, dynamic range, or headroom, which is accomplished by listening to source material and experimenting with audio effects processors.
Equalisation can be performed spectrally, or across individual channels, however most virtual instruments, will have already had excessive resonance(s) removed, except in the case of synthesisers, due to the open-ended nature of sound generation.
Frequency based or multi-band compression, is useful so specific frequency bands can be adapted for systems accentuating certain frequency ranges, or else program material exhibiting excess energy, in specific frequency bands.
Although mastering may enhance a given mix, it must be considered what processing will be applied during mastering, since an engineer must consider the end result, such that some mastering can be applied in a mix, that would otherwise be applied during mastering.