Office Online is the desktop Linux user's access to Office. Enough people use Macs that it makes sense to invest into the effort, else some other vendor will gobble up Office's marketshare. I use Office Online and LibreOffice all the time on Windows 10, some documents it's just better to have guaranteed formatting compatibility with the majority of documents which are put out through MS Office. I would personally just continue the same pattern if I were a desktop Linux user.