Reddit doesn't replace all of search, but for me it does effectively replace search engines when I'm looking for an opinion. Life is short... I don't have time to weigh all the facts about something and make my own decision. While I do that a lot, when getting started with something, an initial opinion to go on is very useful.
Search engines have become really bad at surfacing genuine opinions. Crappy GPT-2 generated blogs have overwhelmed the ability of search engines to surface human generated blogs, and even the human ones are often clearly just someone's hour of research on a topic with the conclusion being "decide for yourself". Sites like Quora make it too easy for someone to insert their opinion without it being adequately tested in the public space. YouTube was once a fairly decent place to hear opinions but, like with regular search, it's currently optimized for the equivalent of low-effort blog posts: videos that are ~12 minutes long with little valuable information.
Reddit has many issues, but at least it's still a useful place to find opinions that are also openly tested against their respective subcommunities. The usual blog type content doesn't last long in most Reddit communities without becoming substantially downvoted. If I want to know why people are choosing a particular programming language, for instance, I can do `site:reddit.com why use c++` and actually get some concise and even nuanced opinions. An equivalent search across all sites on a search engine usually leads to long treatises with little value appearing first above the fold.
If I couldn't use `site:` syntax on DDG or Yandex, I'd probably go directly to Reddit's own search engine a lot of times. So Reddit wouldn't necessarily replace all of my searching, but it definitely would replace a large portion of it.
And I wish that wasn't so. I wish more of what appeared in search engine results could either be Reddit-type content or academic articles as opposed to all the flotsam we get that's somewhere in between those two yet fulfilling the needs of few readers.
EDIT: Somehow I forget how I'll also use HN in a similar way, though less often for some reason.