I'm not quite old enough to have needed to use this while writing my own software. But I've come back to it repeatedly while learning enough about the operation of DOS PCs to try my hand at reverse-engineering some games. That was at least 10 years ago. (It became clear that I'd bitten off more than I could chew, so no full game RE, but I documented a couple more file formats, wrote some file viewers, and learned a lot).