If you self custody any chain fork doubles your stack. You now hold coins on both chains. You can sell the bad fork and buy more of the good fork without any fiat input. Of course choose wisely, previous failed forks are all down 90%+ last I looked. If you have custodial bitcoin you are at the whim of the custodian. You may not get to choose which chain you follow. You may not get to ever access the other chain, the custodian could just pocket it. Yet another reason to self custody.