This looks interesting, can you give a little explanation about how it works?
It basically uses Wallhaven API to query images.
It can either be used with, or without user authentication. Without authentication NSFW is inaccessible, though.
Most of app UI is where filters and sorting (just like top bar in the site itself) located.
The "Image source" section is where needed amount (leave empty if you want to download everything) of images is set, and search query specified (can be empty, just like at the site itself), also from there you can enable "User collection" mode. After "User collection" mode activation you must enter a username of target user, and press "Get collections" button to populate collections list, then select needed collection (to access private collection you, obviously, must authorize). Be aware that in "User collection" mode only purity filter is applied.
After everything is set, simply press "Download" button and wait while images being downloaded.
Download location can be changed by pressing respected button.
This app is useful in cases where you already know what you need to download (like all or some amount of specific search request, or user collection) as there is no any preview.