The regular grip on a pen, for penmanship, works nicely because you are writing a series of strokes, usually in a very linear fashion and usually at a fairly small size. The line moves from left to right, with subtle variations in up, down, loop, change direction, etc. You usually do not rotate the paper.
Offhand flourishing is not linear at all. It is actually drawing. You are building up a series of matching curves that may taper or curve. You keep adding layers. Often times you rotate the paper. It is a picture - not a sentence or paragraph. Compared to words, many of the strokes are huge and the offhand grip gives you more flexibility to make the curves. You can match the look of offhand flourishing using a penmanship grip and the offhand grip may feel odd at first. But, once you try it, you start feeling the graceful arcing motions and it is a very good feeling. Then the practice strokes start looking so much better.
It reminds me of my two boys learning to ski. One was eager to just head straight to the regular slopes. The other wanted to stay on the bunny hill for a really long time. At a certain point, someone had to just take him up on a big slope and tell him to go for it. He was fine and it got him over the hump. So, even though it looks odd and you are skeptical that you can get good results, you might make significant progress with the offhand grip and wonder why you didn't try it right from the beginning.
Back to the size difference. If I am writing and am going to make a huge flourish on a letter, I hardly let my hand touch the paper. I don't switch to the offhand grip, but, I imagine the way your hand is mostly off the paper is another advantage of the offhand grip. It fits well with the whole arm movement.
