usually you should use the write directly to files option.
However, sometimes it happens that the server you're hosted on has permission issues: this means that the current user (server user, not the Joomla one!) has no right to write, leading to errors.
As last resort, Joomla offers the option to enable an FTP layer, meaning that all the write actions are performed with the FTP user you selected, instead of the current server user.
Long story short, do this simple test: go inside the configuration page of Joomla, change any settings and save. If you don't have any errors, you can use the write directly to files option, otherwise you have to enable the FTP layer.
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