What is LibreOffice offline Help Pack and is it necessary?
When you open LibreOffice Help, either using Help buttons, or F1, or menu, LibreOffice first searches for help files installed on your system, and if present, will show their contents; if absent, recent versions will ask you if you want to see online help, or install local help pack available from the LibreOffice site (see Help for offline use).
You may choose whatever fits you better, and either option is good. E.g. I prefer online content.