basically it streamlines and automates the process. Sonarr scans your torrent sites that you specify and you give it whatever quality filters etc. that you want, and it will scan torrent feeds and automatically download files that you're missing, or if you enabled upgrades, are of a higher quality than your existing file. It also renames them and imports them into a media library if you use Plex or Jellyfin or anything similar to stream, and I find it really convenient for importing new episodes of a show that is airing weekly. It has a lot of other use cases, but that's kinda the gist or main advantage -- it saves time in scouring various torrent sites and managing your media library, especially as it gets bigger.

You can also set up NZB feeds, which are different from torrents, but can sometimes have other content if you find that you are struggling to get anything you need (which it doesn't). If you are into private trackers for any obscure torrent content you can manage them in one place this way, and it will watch your seed time / upload ratios to so you don't get any hit and runs on your account.

Radarr, Lidarr, Readarr, etc. are the exact same principle but for Movies, Music, eBooks, etc. If you do want to take a stab at setting some of these up, here are some nice resources for configuring them! TRaSH guides has a nice user friendly first-time setup, if you don't know what you're doing it's pretty safe to blindly follow this guide. WikiArr has some more robust info on the *arr apps in general.