How are youtube-dl and yt-dlp different from Online YouTube Downloaders?

What is this sub about basically ?

youtube-dl, and yt-dlp are a command-line program to download videos from YouTube.com - is what the sub description says. How is this different from online tools,FOSS apps like YTDLnis or Seal which can also download videos from youtube . I am genuinely curious because i often download youtube content.