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 ?
In previous years, Bitcoin has always given a good profit increase for the year, only this year it probably will not give annual profit. As long as Bitcoin grows in price in general, until it can be held for a long term, when its price stabilizes, and sooner or later it will happen vseravno, then it will be useless to keep it for a long time.