This function starts the GRASS GUI. It is provided merely as a utility... in most cases, it should not be used if you are doing any kind of analysis of rasters or vectors using fasterRaster. The reason for this prohibition is that fasterRaster objects, like GRasters and GVectors, are really "pointers" to objects in GRASS. If fasterRaster points to a GRASS object that is changed in GRASS but not R, then fasterRaster will not "know" about it, so changed won't be reflected in the fasterRaster object.
One aspect of the GUI that is useful but will not change objects is to use it to plot rasters and vectors. However, the a fasterRaster object in R will have a different name in GRASS. The name in GRASS of a GVector or GRaster is given by sources().
Examples
if (grassStarted()) {
# DANGER: Making changes to rasters/vectors in the GUI can "break" them in R.
if (interactive()) grassGUI()
}