Salem

Does high purity hay make any difference in anything? Or just use it like normal hay?

It's not really useful except for a few select inspirationals, like the WWMM.
I'm storing my remaining high purity hay for now though, who knows if the devs think up something where it matters, like ore smelter purity

Back in the day before jorb and loftar broke the game it had some uses.

1) When you didn't have 70%+ pumpkin seeds yet and wanted/needed to level perennial philosophy you needed some to make 10% wicked wicker man masks.

2) Also used with aspens to make high thread and needle inspirationals

Now it doesn't, but I'm sure eventually it will be used in some kind of formula to determine the structural integrity of your house and if it isn't high % purity enough your house has a chance to implode and KO+murder you while you're inside losing your character forever. Just another wonderful update on the horizon.

any item used to make inspirational is better with higher purity, since it gives you more points.