What is the benefit of using Cas9 protein instead of Cas9 plasmid or Cas9 mRNA?

Cas9 protein is a DNA-free option so there is no concern of DNA integration that can happen using plasmid. The protein is immediately available for the cell to use for editing as it does not require transcription (as for plasmid) and translation (as for mRNA). Additionally, because it is quickly turned over by the cell there may be a lower chance of off-targets.