Reddit also offloads the complexity of forum management from moderators and removes the friction of registration.
If you want to create a traditional forum, you have to figure out how to host it, what forum software you want to use, how that software works, how to configure it, etc. If you get attacked by spammers, hackers, or trolls, you have to figure out how to stop that more or less from scratch. It requires a significant amount of technical knowledge, time, and money to make it work.
Then, even if you use a dedicated forum hosting service, your forum is still basically on an island. You have to find a userbase and attract them, convince them to register dedicated accounts for your forum specifically, and keep them coming back, which is really hard if you don't have an existing userbase to make your forum compelling in the first place.
With Reddit, nearly all of that goes away. Creating a subreddit is extremely easy and costs nothing but time. Moderators still have to moderate, but they don't have to figure out how to manage forum software or handle DDoS attacks. Since any Reddit user can join and participate in any subreddit, and since posts appear on users' homepages in one feed, communities can grow much more quickly and are less likely to die out due to inactivity. And there's only one UI for the whole site, so users don't have to learn how to use whatever random forum software each site has chosen.
What, you don't want to sign up for 27 different forums, all using their own variations of CRUDs?
The solution to Reddit is a similarly centralized approach managed instead in the style of Wikipedia (not suggesting exactly replicating their governance, rather something generally akin to Wikipedia's not-for-profit direction).
If you want to create a traditional forum, you have to figure out how to host it, what forum software you want to use, how that software works, how to configure it, etc. If you get attacked by spammers, hackers, or trolls, you have to figure out how to stop that more or less from scratch. It requires a significant amount of technical knowledge, time, and money to make it work.
Then, even if you use a dedicated forum hosting service, your forum is still basically on an island. You have to find a userbase and attract them, convince them to register dedicated accounts for your forum specifically, and keep them coming back, which is really hard if you don't have an existing userbase to make your forum compelling in the first place.
With Reddit, nearly all of that goes away. Creating a subreddit is extremely easy and costs nothing but time. Moderators still have to moderate, but they don't have to figure out how to manage forum software or handle DDoS attacks. Since any Reddit user can join and participate in any subreddit, and since posts appear on users' homepages in one feed, communities can grow much more quickly and are less likely to die out due to inactivity. And there's only one UI for the whole site, so users don't have to learn how to use whatever random forum software each site has chosen.