Like UNI token...Apart from its use as a governance token I can't really understand why the fuck people buy it? Just because DeFi is hot right now?

Uniswap does around $1 billion in daily volume on the platform. With the 0.3% trading fee that means people are spending roughly $3 million every day on fees just to use that platform. While this money doesn't currently go to UNI token holders, it will likely start to in the future as UNI holders vote what to do with fees. Many defi governance tokens are similar. There are a lot of tokens that have no good reason for their high valuations, but I don't think UNI and the other popular defi tokens fall into that category. There is a LOT of money going through those platforms and generating fees and that is likely to grow significantly in the future.