Why is the ability to convert files relevant for the relevance of formatting text?

If my professor wants a double-spaced document with Times New Roman in 12 point font and 1 inch margins, that’s very easy to do with a plain text file as input (They can load a template with all their defaults in their WYSIWYG editor and paste my text in). If I, as a user, want to convert a formatted document back to plain text to read/edit it (which I prefer), this is unreliable, and can lead to malformed output, especially with proprietary formats like docx that contain images; or flat out impossible with other proprietary formats.