Because if you only hold Bitcoin your balance will grow up only on USD rate. For example you hold 1 BTC . Current price 7500$. It's might 10k after some day. But you still have only 1 BTC.
Now think you have 1 btc and you convert it into BCH. Example for 1 BTC you get 10 BCH. Rate per bch is 0.1 BTC .
After some day if Bitcoin grow up, At the same time BCH also will grow up.
If bch rate grow 1.5 btc then you will have 1.5 BTC.
But before buy alt coin please select best coin to avoid dump.
So what you think. Hold alt coin or Bitcoin ?
This has already been thought of earlier. This is the reason why even if I am an avid fan of Bitcoin and of hodling it, I always make sure that I am also hodling the top altcoins. This will be a strategy for your portfolio to remain good even if either altcoins or Bitcoin is going down. But if the entire market is in blood, you cannot do anything about it anymore. And then, I would also like to add that you make the basis of your portfolio's value in BTC and not in USD.