It is, in short, a resource cache. I put it in the project (by default, there is a setting) to make it visible – you can commit parts or the whole directory to Git, if that fits your style.
- If you are theme author, you will most likely want to put the generated/fingerprinted resources as part of the theme distribution, so the users does not need to have PostCSS etc. installed to run your theme.
- If you use lots of thumbnails, you may consider comitting this to GitHub to avoid having to create thumbnails on every CI build etc.