I started this project because most of the generators and templates I saw out there are really geared toward entry and junior-level professionals. I've been working for over 15 years now, and have a ton of job records since I started out in consulting/contracting. So my own needs were to
reduce verbosity in my resume. That's why I added extensions to the default schema, that allow you to list older jobs with less detail than more recent work history, etc.
Like you, I've been maintaining the same Word document for years now. A pain point for me has been tracking changes over time. Like I said, my work history is pretty extensive... so every couple of years I go back and trim some detail off older listings, to reduce length and prevent my resume from looking like a CV. However, I hate to lose that older content forever, and storing multiple versions of Word binaries and diff'ing them is kinda clunky. Separating the content from the style really helps.
Also, earlier in my career I had to maintain two or three different versions of my resume, to target different types of job opportunities. If I were applying for a more front-end focused position, then I had one resume that put more emphasis on JavaScript and HTML experience. For back-end focused positions, I had a version more focused on Java and SQL. At this point in my career I have much clearer vision of my career path, and know what I'm looking for in my job hunting. Also, we're ALL "full-stack" now, right? (just like all companies are "agile"!) However, if you do need to maintain customized versions of your resume content, then being able to branch in git is awesome.
Lastly, I just like the ability to switch up the look-and-feel every so often, without a ton of cut-n-paste hassle. These days I spend more time on the other side of the interview table as a hiring manager. You CAN go too far off the deep end with silly resume design, but I'm here to tell you that a little touch of style definitely makes you stand out from the crowd. Even if it is a canned template. 90% of developer resumes today are ugly blobs with no real formatting at all.