I guess you could ask why does anyone use karma/likes/upvotes (even fiat currency).
That said in the beginning not many people wanted to "use" bitcoin, it was initially embraced by crypto enthusiasts and even then the first bitcoin had to be given away for free. There was no million or billion dollar ICO, it was given to the community for free to start. The first actual pruchase on record was 2 papa johns pizzas for 10,000BTC (~$50M at the BTC peak).
What makes cryptocurrency (not asset backed tokens) valuable is up for debate, but generally people agree it's: 1. Limited supply, 2. Immutable blockchain technology, and 3. The network itself. Thereafter it's just belief. Now I'm not claiming YCcoin will compete with Bitcoin, but I think it's one of the coolest applications yet of a community cryptocoin, but I'm happy to hear suggestions.
I think the HN community could come up with more reasons than I can envision, but I can start with giving YC Coin to high quality posts/comments, "funding" show HN projects. YC Coin can already be traded on one exchange for USD, Euro, BTC, ETH (not that it has yet, I'm still trying to give it away free initially).
