https://gdal.org/drivers/raster/ers.html
does not in any way do it justice.
What it allows you to do is create a text header description for a binary file .. pretty much any binary image (and not intended to be image) "raw" (ish) format that existed then and now.
Band interleaved, band seperated, row interleaved, column orientated, etc. 8 bit, 12, 24, 32 bit, big or little endian, with or without header "junk" to skip at start of file and|or at the start of each "virtual row"
The handy hack was the ability to view as image and file that had time series aquisition by sketching out how to block it, how to understand the binary, insert optional colour maps .. and then being able to rapidly visually scan 2GB or 16GB of binary blob with real time fly about zoom in zoom out capability.
Intended for multichannel satellite | aircraft | other instrument data with a variety of metadata (timing, position, instrument orientation, etc) - useful far beyond its core target.
Addendum: for the curious, somewhat better description of the .ERS layout from PDF page 33 onwards of https://www.aseg.org.au/sites/default/files/ER%20Mapper%20Cu...