Aside from the technical explanation that others have given, here's how I use Docker:

MeTube to rip videos and stuff easily. Just plug in a link and most times it'll work. Here's a list of all the supported sites.

I use Sonarr/Radarr and qBittorrent with gluetun to search for and download TV and movies that I watch on Plex.

I host my own Immich server that will automatically back up my photos from my phone just like Google Photos, except I own it all and it's all kept private. It has its own machine learning and facial recognition, so I can search for "dog" and get all the pictures of my dogs, or I can search by person.

I use Docker for all this because the images come in little prepackaged containers. It's super easy to get into once you figure out some of the basics.

Another great benefit of these containers is that you can transfer it to another system if needed. Just copy the config and data over to the new system and point the container in the right direction and it'll pick up where it left off.