Docs say:

    If set, the transcription output will be sent to the specified file or URL
    (use one of the FFmpeg AVIO protocols); otherwise, the output will be logged as info messages.
    The output will also be set in the "lavfi.whisper.text" frame metadata.
    If the destination is a file and it already exists, it will be overwritten.

    @item format
    The destination format string; it could be "text" (only the transcribed text will be sent to the destination), "srt" (subtitle format) or "json".
    Default value: @code{"text"}

Of course, you could already do that by just manually calling whisper on files, but now you don't need to export parts or transformed media files to feed into whisper.