In my experience, private instances of PhpBB forums provide niche webtools that are necessary for community building that Reddit could never replace.
The Tetris community created a Javascript gui called Fumen (https://harddrop.com/fumen/), that can be embedded into PhpBB forums.
The chess community created Javascript GUIs that read/write .pgn files (aka: move-history files) to help the community discuss games.
The Go community created a similar GUI for Go games. Etc. etc.
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Since the administrator controls the whole webpage (including Javascript), you can host local tools in a Reddit-like environment. I think people are sleeping on the potential here, especially if you've never used these custom web-GUIs before.
The Tetris community created a Javascript gui called Fumen (https://harddrop.com/fumen/), that can be embedded into PhpBB forums.
The chess community created Javascript GUIs that read/write .pgn files (aka: move-history files) to help the community discuss games.
The Go community created a similar GUI for Go games. Etc. etc.
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Since the administrator controls the whole webpage (including Javascript), you can host local tools in a Reddit-like environment. I think people are sleeping on the potential here, especially if you've never used these custom web-GUIs before.