1. Real-time user feedback. Discord is conversational. If someone gives feedback on your product, you can quickly have a conversation about the exact issue they're facing and you can propose solutions to see how they feel.
2. User retention. When users have such immediate access to the product's developers, it makes their opinions feel helpful and useful and drives user retention.
3. User collaboration. Once your community gets big enough, users start to collaborate together. This might be directly in the product (if it's a collaborative product), but can also take the shape of brainstorming improvements and feedback.
4. Community help. Over time, members of your community becomes experts in your product and they help newer users onboard. This frees up a lot of dev time that would have been spent providing support.
Personally, I find the opinion that "it's not indexable" a little silly. How many times have you searched an arcane error and been brought to a mailing list? More often than not, the indexed email never received a reply. If it did, the answer is unlikely to solve your problem. Combing through conversations is not an efficient means of documentation. We use our community feedback to get a lot of signal about what's missing from our documentation and then update it accordingly. 10 users came in this week asking about feature X? That means we need to update our documentation, etc.