Hi,
Even with the Checkout set to 'NO' you will get the option to check out the file and checkout the file.
The purpose of this feature is to only allow one user at a time to modify the document who will be prompted to checkout the file before editing the file. At the same time no one else will be able to edit the file.
For further explanation please refer following article:
Check out, check in, or discard changes to files in a library
Hope this helps :)