Simultaneous working papers strategy
If you work with coauthors, who will all have their own conflicting responsibilities, there will frequently be times when someone cannot work on a paper and the others cannot meaningfully proceed. In such times, you switch your attention to a different paper you are working on (alone or with other colleagues).

So, unless you work alone (not recommended), the "parallel working papers" strategy is less of a choice and more of a necessity. I personally am currently trying to juggle a textbook, two coauthored and one single-authored paper.

The same will apply to grant writing.

And even if you do work alone, there comes a time in a project when it's useful just to take a break and do something else for two or four weeks, then return to your initial project with a fresh mind.