According to Etsy, Treasuries are ever-changing, member-curated shopping galleries.
When a Treasury is particularly cool can be chosen by Etsy editors and be on the first page for a while.
And that is probably the best form of advertising you can buy.
Of course, for a chance to have your collection featured on Etsy's home page, you have to follow some rules.
I see Treasuries as another form of crafting, since most of them are notably beautiful.
Above, A Valentine's day collection
For those who loves Etsy and join its community, Treasuries are a pleasant way to interact with the others and with the platform. Not only they offer visibility to featured sellers and to the curator, they make them meet each other.
Nevertheless, everyone who sign in can arrange a Treasury List and I think you should do that once in a while. It's very funny. You can think of your treasury as your own wish list.