Specifically, they are useful for stripping comments, compacting huffman tables, and slightly reducing quailty to make them smaller. You can vary the quality setting for higher quality with less compression ratio, though . Jeff Frohwein Framage is a tool which converts large bitmap images containing multiple animation frames into multiple images each containing a single frame of animation. You can run the resulting image through hackit and you'll have a raw file perfect for loading into sprite ram to use for linear mapped sprites. It also works under WINE for linux.
Source code is available as well.
A 20 minute demo of using this package may be downloaded as well. Tiles are input in DeluxePaint-style format. DP is no longer made but Pro Motion is a DP clone for windows. While Open tUME is now freely available, they still offer paid support for the same price as before. I discovered this tile and map editor which appears to be suited for GBA development if someone produces an export filter for it.