Thinking more about it: Not asking "why use X instead of Word", but asking "Why would you use Word instead of anything else?"
Only reason I use Word is because I from time to time get some strange formated and macro filled document from a client who lives Word and I need to use Word to properly read the document. -> so the only reason I'm using it from time to time is because I'm forced to do
Most of my stuff I write in a normal text editor (and save it inside a git repository ;)).
I only use Word/LibreOffice/GoogleDocs/... for formating and to meet a clients needs.
For personal stuff, which needs formating I normally use LibreOffice.
For publishing stuff I use latex.
-----
Bottom line is, I see no reason to use Word over any other tool.
Even if LibreOffice, GoogleDocs, Office 365 and Word would all satisfy my needs, there is nothing that Word offers to me that separates it from the rest.
-> Why use Word?