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 ?