It is using its mast-mounted colour cameras and laser spectrometer, together with its arm-held "hand lens" camera and X-ray spectrometer, to try to find the best drill candidate.
BBC: Curiosity Mars rover deploys its rock brush
2.
Brushing the surface of rocks with Curiosity's dust removal tool first enables the arm-held hand lens camera and X-ray spectrometer, in particular, to see features that might otherwise be obstructed.