For curation. For categorization and deduplication. And for trust.

You can only express so much with titles and seed/leech counts. Trackers that were known to be the best - like What.CD and waffles.fm - were private and heavily moderated. You had to be invited, usually by passing a quick interview to prove you knew how to avoid common data management pitfalls like transcoding from a lossy codec. That made those trackers into carefully curated collections of high quality content.

Even public trackers benefit from some moderation. Most people only want to be browsing porn intentionally, so having a category for it let's you browse for movies or whatever. It's all about sorting the noise into signal.

It's also helpful to have some commentary on a torrent. Maybe the subtitles are forced. Maybe the software has a virus or includes adware, but ended up getting a critical mass of seeds/leeches anyway. Maybe there is some similar content worth talking about.

Have you ever used a public decentralized platform that wasn't full of trolls? I haven't.