I think it depends. If you are in pre-production and have the time, it would be great to organize your thoughts in a coherent manner. That becomes more important if other people are expected to follow your design vision. But also good for you if you want to remember what you planned 6 months ago without having to reinvent the wheel.
Some games won’t need a design doc though.
And sometimes you may be going so far off the beaten path that you will simply have to make things up as you go.
There are plenty of ways to do a design doc that do not follow that traditional doc standard. I have a 30 page(unfinished) design doc for one game concept, but my current project is more akin to a couple dozen excel spreadsheets and assorted word docs.
I don’t remember a lot of famous quotes, but one that has always stuck with me is this:
“In preparing for battle I have always found that plans are useless, but planning is indispensable.” - Dwight D. Eisenhower