I know Reddit search isn't great, but the ability to search at all is useful. Compared to e.g. Discord where threading is weak at best and searching is borderline useless because it's a chatroom, not a forum.
Reddit is a low-effort replacement for traditional forums, solving several problems of traditional forums:
1. One account for all communities. Nobody wants to make 100 accounts, whereas it's really easy to just click "join" and start participating in a new subreddit.
2. Consistent, uniform, familiar interface with Markdown for text. The old PHPBB etc. forums all had different layouts and slightly incompatible markup. And Discourse is just weird.
3. Easy to stay "casually engaged" with the community because of the customized per-user front page and the ability to group communities together with multireddits. Whereas with individual forums, you need to get email notifications or manually check several different sites to stay connected, which is much higher-effort. Also, it's easy to passively follow a low-population subreddit in the hope that it gains more users, while you're much less likely to make a new account on a forum that seems quiet. This helps keep the appearance of a high active population in niche communities, because people are less likely to forget about the community and can easily drop in and out.
4. Easy discovery of new communities and being a name brand makes it easy to attract users: people just assume a subreddit for X exists. The alternative is that they'd have to hunt around for a forum that will probably be low-population anyway.
5. Upvoting and downvoting helps the community moderate spam and bad actors. It also creates incentives to post content, in the hope that you get upvotes. Yes, there are problems with the system, but I think "the market has spoken" to some extent on this: the individual enjoyment of voting outweighs the systemic incentive problems associated with it.
It's easy to say that Reddit is "just" the two things that you mentioned, but that to me is reductionist to the point where it obfuscates why people prefer Reddit for discussion compared to other options.
Note that Discord has many of the same advantages over its alternatives, and I think similar points are relevant for both platforms.