There's no such "benefit" of joining the discord. It's a platform where you can meet all the creators around the globe, have a chat with them in general chat, discuss youtube related or TubeBuddy related talk or share your thumbnails for feedback or promote your videos and can even look for collaboration and every Friday, you can be the lucky one to be selected for the Feedback Friday Channel Review or SEO Review.
Just like the forums, you will be monitored by the Trial Moderators and Moderators and it's pretty much similar to the forums, the only difference is that it is a bit more "Advance" and a more informal way of the platform. Just you need to ensure that the rules are followed and rest, you will be doing just fine.
You can join the Tb discord right here
https://discord.gg/tubebuddy