But the good news is that it’s a completely different story with token-to-token transactions. Coinbase doesn’t charge you any markup fee — but there’s some inevitable spread. And with some obscure trading pairs (exchanging ZRX for BCH for instance), you might end up paying around 1 percent in spread. Still, it’s a much better user experience for those who just want to trade on Coinbase. Without even mentioning other exchanges, Coinbase Pro users have been able to trade between multiple cryptocurrencies for a long time. But Coinbase is still the entry gate for many new cryptocurrency users.

https://techcrunch.com/2018/12/18/coinbase-lets-you-convert-one-cryptocurrency-into-another/