This program lets you put one image into the same voxel space as another image. E.g. you might want to extract a Brodmann area from the brodmann.img file that comes with MRIcro, and then put that set of 1x1x1mm voxels into the same voxel space as your functional data, which probably covers a smaller volume and has larger voxels. Then you can use the Brodmann area as an ROI to explore your functional results.