Sometimes you want ffmpeg to ask you whether it should overwrite a file. Sometimes it's just a script that you prefer would fail if something is amiss. I.e. don't rely on stdin to answer if you have a question.
See https://ffmpeg.org/ffmpeg.html#Main-options
-stdin -
Enable interaction on standard input. On by default unless a pipe is detected.
-nostdin - To explicitly disable console interactions. Without -y this will cause ffmpeg to error out if the target file exists.
-y - To overwrite the output file