My analigy:

You have a Reddit account. You recently bought a Honda Civic, and know there’s a web forum for civic owners over there, called hondacivicforum. You would like to participate in it.

You can just subscribe to the forum topics you care about from your Reddit account. No need to create a new account.

Also, you have family on Facebook that posts updates and photos and whatnot. You can follow them, too, and reply to their posta without needing a Facebook account.

You use Reddit. You can interact with content outaide of Reddit from Reddit.

Tada.