help to understand files in sites/all/default/files/js folder I'd like to understand what are the ....js.gz files located in sites/all/default/files/js.

I thought they are cached js files. 

But if I clear all caches (drush cc all) these files still remain there. 

I've made a capture of the folder:
Like you said these are just cached versions of the javascript files. Drupal compresses them so they can be served to the user faster, notice the difference in file size between the version 29.1 KB opposed to 117.4 KB.

Are there any error when you clear cache through drush? Mostly I've found that they stay behind when you run drush as a user that cannot modify the files, e.g. a user not in whatever group owns the files. In this case you need to add your user to whatever group owns the files.