Reduce the size is useless since the .js and .css files are not downloaded unlike a website.
How important is the improve of performance with min file? (< 2% I think)
Lots of libraries, frameworks or components are distributed with a .js and a min.js version. They are functionally the same, just the .js version is readable but the .min.js has been minified to reduce size. If you include the .min.js versions you can remove the .js versions. You still need to include any .min.js versions in your index.html script tags though.
Yes probably irrelevant in 99% av the cases. But as a project grovs big it is sometimes desirable to make the app package small to download from the appstore. And sometimes the only way is minifying a lot of js files.