Reformatting manuscripts for a new journal is, in my opinion, one of the most tedious parts of being a scientist. I won't say that it is a total waste of time, but it comes close.
I do understand that each journal has a unique format, structure and feel. Each journal wants to make its format consistent for its readers. And, each journal has a slightly (or vastly) different audience that its format caters to.
But, sweet, mother of pearl, it can be frustrating for authors to update, change, add, and repackage information for a new journal. This one allows sub-headings, another does not. This one allows you to merge "Results" with "Discussion", another does not. Some journals and articles have specific word limits, others let you include as much content as you like (although some charge by the page). There are varying limits on the number of tables and figures. Even different pricing for color within figures.
I am very lucky to live in the digital age, where technology exists to easily cut, paste, add, and subtract content. My dad tells me a story where, for his Master's Thesis he literally cut out, then taped in new content, because otherwise he would have had to retype the entire tome.
And citations, whew! I am so glad for the software to assist with updating these. There are so many different formats for citing in the text (only the first author's last name, or up to three authors' last names, or numbered by the first time the citation is present, or some other variant). The bibliographies can be equally involved regarding the small, but important, differences between journals.
Given all of the changes, and the amount of time, that go into choosing to submit to a journal with a different format (which means that, unfortunately, the manuscript has already been rejected from one journal), there are some good things. A new format does require the author to consider their analysis and results from a new structural perspective, considering how to tell the story in a new way. Hopefully the process will also give authors the chance to catch any minor writing errors, and think of new, clearer ways to explain observations.
But, I'm not entirely convinced that those benefits outweigh the time spent trying to wrangle a manuscript into the appropriate format.