The point is to force Google towards a site you expect may have the result you want. site:news.ycombinator.com works for similar reasons.
Basically, you can find (likely organic) discussion of topics by groups who are familiar with the subject and are likely to quickly correct any incorrect or misleading statements.
Otherwise, Google will believe that some no-name site with generated content is... somehow... a good search result. The 'reddit' approach works poorly for me recently, Google gets stuck in certain sub-reddits and there's a lot of low quality or bot content on reddit to filter out anyway.
Basically, you can find (likely organic) discussion of topics by groups who are familiar with the subject and are likely to quickly correct any incorrect or misleading statements.
Otherwise, Google will believe that some no-name site with generated content is... somehow... a good search result. The 'reddit' approach works poorly for me recently, Google gets stuck in certain sub-reddits and there's a lot of low quality or bot content on reddit to filter out anyway.