What is the purpose of the i18n/en_US.csv files within modules? As in... how are they specifically used?
I realize it's for the purposes of translations, but I have tested adding translations for another language (ex: Italian) in an i18n/it_IT.csv file and it translates as expected regardless of if the i18n/en_US.csv file is in place. Is it actually being used? If so, can someone give me an example of when it would be problematic if that file was not in place or missing some phrases in our modules or theme?