Originally Posted by
YogSoggoth
The hat is supposed to have an on-use effect of increasing your fishing skill (or at least used to have it before), but after 9.1.5 or something the "cosmetic" tag has been added for it, making it unusable. As soon as you rightclick the hat, it gets consumed and added as a transmog option. Not a big deal, there are other items that fishing.
Just kinda strange for Blizzard to add the "cosmetic" tag for an item with an on-use effect, since they also added the tag for a whole bunch of holiday event themed items, negating their on-use effects, but also substituted them by adding some toys with the same effects that you can now get from respective holidays.