Okay so I just built a Ruth Stout style garden bed, just some kraft paper from a roll laid down and then hay on top.

But it occurred to me that all the youtube videos and everyone I've seen, fluffs the hay and spreads it out. Is there a reason why you don't just leave it in flakes? Seems like it would be more dense and weed choking that way, and it compacts over the winter even if you fluff it.

I ended up leaving it in flakes. It was easier that way and I figured I'd inhale less dust and junk. But I wanted to come in and ask. I did take a few last of the flakes and kind of fluff them around a bit but I figure since I put them on overlapping a bit it wasn't like I'd have seams where weeds could come through.

I guess I never noticed that Bethany.  You might take some observations of what your garden looks like next year and see if you can see a difference.
I can only guess that it makes the hay go further, covers more area , if you fluff it up ? Maybe it composts faster ?

I don't recall what she said about establishment, but Ruth Stout herself recommended just throwing whole flakes down on top of weeds during the season.

I suspect fluffing the hay during establishment is more or less an economic consideration.  By fluffing, you cover more square footage per bale.  Then you can target problem areas with the denser flakes.

I think that it is probably a combination of four reasons.

The straw, as mentioned, definitely goes a lot farther when separated.

When straw is fluffed up, it gives an aerobic benefit to the mulch system and to your soil, whereas the blocky compressed bail flake is largely an anaerobic system unto itself.

Fluffing it up allows a massive amount of ecosystem for spiders and other beneficial creatures, whereas large blocky flakes allows a minimum.

Fluffing it up gives surface area for dew collection, and rain or irrigation penetration.

I think you would end up with a more uniform coverage in your mulch by fluffing the hay a bit so that everything is more homogenized and locked together.  It seems like the edges of the flakes wouldn't lock together very well and give an easy route for grass and weeds to come up through the paper and mulch at those seams, if that makes sense.  As the fluffed hay compacts it intertwines giving you a better sheet layer instead of the tile effect you might end up with using the flakes.  

Both Dave and Roberto have given the reasons for "fluffing" straw for mulching.

Most people want a mulch that covers evenly with no gaps to allow weeds to gain a foot hold, Whole flakes will leave such gaps.
Loose mulch that can then compact over time is very biological friendly because of the air it allows to get inside.
This means no anaerobic bacteria in the beginning, plus lots of room for beneficial insects to make their homes.
It also allows you to build the depth you want to shade out undesired plants, water to infiltrate easier, support for some plant species that start off with thin stems that later on thicken to support the growth.

Redhawk