Why is seeding a torrent helpful?

I keep seeing in forums and sites like these that say it's frowned upon to not seed torrents that you use/used. I saw a post on here or Reddit (I don't remember) with a guy ecstatic that someone started seeding his download he had been trying to get done for months. I know seeding lets someone download something using your computer but how is it helpful if someone doesn't have a site and/or isn't "in-range" ?

If you can't tell, I don't know much about how torrenting works other than how to download something using one. I hope that you all can just explain or point me in the right direction because I would like to support the community.