It's there because according to microsoft's documentation, this registry key prevents windows from automatically making calibre the default handler for a file type. The documentation also explicitly states that it should not prevent the entry from appearing in the open with menu, see
https://docs.microsoft.com/en-us/win...ell/fa-progids
Apparently your experience indicates that documentation is wrong. I added that registry key because sometimes windows would make calibre the default handler for one or more of the filetypes it registers for and then people would come here and yell at me, accusing me of taking over their filetypes associations.