This is a very cool idea and the use case is described perfectly by the authors:

Does your designer cry every time you load their beautifully designed screen, and it is full of empty boxes because all the images have not loaded yet? Does your database engineer cry when you want to solve this by trying to cram little thumbnail images into your data to show as placeholders?

You give BlurHash an image and it gives you back a short string (20-30 chars) that represents the placeholder image. Store the string and send it along with the other data for the object and your client can decode the string into a placeholder image while the real image is loading over the network.