HBD is the base asset for all trading pairs for the time being. But you can easily do a BTC->HIVE swap or a HIVE->BTC swap, its all routed through HBD.
When 1 base asset is used in all trading pairs like this that significantly increases liquidity efficiency as its spread across much fewer pools.
For a 100 different coins pairing them with each other like Hive/BTC, you would need 5000 pools.
If you use a base asset for a 100 coins only 100 pools are needed, liquidity isn't spread thin and the base asset (HBD) becomes the liquidity backbone.