People use scripts to reorder their lessons for many reasons! Most people trying to go full speed use them so that they can learn the radicals and kanji as soon as they become available, then spread out the vocab lessons over several days while they’re waiting to guru the kanji. It’s a way more sustainable study schedule then trying to binge 70 lessons at the beginning of a level to get through the old level’s vocab as well as the new level’s radicals and first batch of kanji all at once.
However, there’s a danger to this: some people get too tempted by the power the script gives them, and end up skipping their vocab lessons and just learning the kanji. This is generally a bad idea because WK only teaches one or sometimes two readings with the kanji, and then teaches the rest through vocab. When people caution against reordering or talk about the dangers, this is what they’re referring to.
But trying to go fast isn’t the only reason to reorder your lessons! I am very much not trying to speedrun this, haha, and the method I mentioned in my post is something that I like to do because I don’t like learning kanji in huge batches, and I get demotivated trying to chew through a bunch of old vocab lessons at the beginning of a level before I can start learning new content. So spreading out the kanji lets me learn kanji every single day except for the last few days of a level while I’m waiting for my final kanji to guru (I let myself take it a little easier then and do just 10 vocab lessons a day until I level).
There’s a more in-depth description of using scripts and controlling your level up speed in the ultimate guide to WK, which I recommend reading if you haven’t looked at it yet 