A while back itag 18 and 22, MP4 video that includes an audio track, seemed to disappear in favor of split video and audio, e.g., formats with video only like itag 136 and 137. This requires using ffmpeg to merge an audio track with the video. More recently, it appears they have brought back itag 18.
This requires space for two files, the video file and the audio file. After merge, which is a separate step from download (the "dl" in yt-dlp), the extra file is deleted. Whereas with itag 18, only space for one file is needed and no extra merge step using ffmpeg is required.
