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A blurhash is much easier to send along with the JSON data used to display the initial page. To see a blurred version of a progressive JPEG the browser must have first started to download that JPEG. When there are many JPEG's to download, it may not even have started downloading yet.

I typically store it in the DB along with the image ID (or URL). It's being displayed even before network request to the actual image has started. Works like a charm on slow networks.

Store what? The BlurHash ?

Correct. It's just 20-30 bytes of data.

Mastodon, for example, use it to defer loading of images tagged with Content Warning until the user clicked reveal. It provides a useful context for user to decide whether to view the image, while not require browsers/clients to load the image beforehand.