This is not a true statement, usually when bitcoin shows an increase, all altcoins fall, because money is being transferred to bitcoin. But when, after growth, bitcoin starts flat, then altcoins begin to show growth.
When Bitcoin raise, the popularity of cryptocurrency raise too. People pays attention not only to Bitcoin but to altcoins too.
Bitcoin is more stable than altcoins. Thanks volatility of altcoins it is possible to earn more money on altcoins.