Occurrence
For example, an old map or field control points are in latitude-longitude WGS84 (degrees) but you want to display and work with the image in UTM GDA94 Zone 55 (meters).
Solution
You will have to follow a 2 step process to produce an image in the desired projection:
1\. Rectify Image
Use the Discover->Images->Rectify Image tool to rectify the image into the projection of the known control points (e.g lat/long WGS84 from example above)
2.\ Reproject Image
Use the Images->Reproject Image to then reproject the rectified image from the its projection (WGS84 in example) to the desired projection (e.g. GDA94 Zone 55).
Note: change the default output name of the Reproject tool to a more concise name, rather than inputimage_rectified_reproject