When working with non text file formats (e.g. coding a parser or file generator) hex editors are convenient for checking that data are where they're expected to

When working with non text file formats (e.g. coding a parser or file generator) hex editors are convenient for checking that data are where they're expected to be, finding out where they are otherwise, or checking what values they have. Even compressed file formats have header information that can be inspected if you have the format's specification.