They were formats designed to be fast on floppy drives which were major storage format at the time, and most importantly, designed as "work in progress" format.

They were formats designed to be fast on floppy drives which were major storage format at the time, and most importantly, designed as "work in progress" format. For interchange, you were supposed to use other formats - for example RTF with Word, which was kept in sync with DOC capabilities all the way to Word 2003 which was the last version that used DOC (2007 uses DOCX and no longer maintains DOC+RTF combo in sync with internal features). However, saving back the file if you did small change in large RTF file took ages in comparison.