your own coins are interchangeable with each other. that does not make bitcoin fungible. i give you an example: you could own a bitcoin in your utxo which was used by another person for buying a fake id on onion and it is marked. in some situations it would be treated differently than a freshly mined or bought btc because it is clearly trackable and traceable. the good thing with bitcoin and any other decentralized coin is that it cannot be confiscated or stolen (if you self-custody and know how to protect it). but it is not fungible and as such it is not interchangeable and not treated equally, you could face some problems in acceptance in certain places. a fungible coin can fix this