So, a volume has one value per voxel. Those discrete points in space, located within the volume's FOV, make up a data set grid. When you rotate the data set, what is its "new" grid going to be? It is the same grid as before, but with data values "rotated" through it? If so, you will have to interpolate the new data values in some way. That interpolation is resampling (which can be done any number of ways: NN, cubic, wsinc5, etc., depending on the data set).
Again, typically afni_proc.py deals with this for you; transforms *are* concatenated before applying, so that multiple resamplings/interpolations don't take place.
-pt
