couple things:

• generally the rules prevent torrents from becoming incomplete
• usually theres a request system, which can be very useful for old or obscure stuff.
• usually much faster for all but the most popular stuff
• way more timely and consistent. Less problems where it takes a few days after a release for it to show up or for a series to stop being uploaded half way through a season.
• less chance of honeypots or other malicious behaviour (you should still use a VPN though obviously)
• more consistent quality and naming enforcement (which means it usually works better with *arr)

I can’t vouch for this particular tracker but in general (free) private trackers are very nice.

it’s mostly to find underground stuff like festival movies, old foreign tv shows etc

I guess quality and sometimes s(p)eed for older/niche pieces, also no dmca bots tracking private trackers so in theory you don’t need a vpn.