When paying the trading fees on Bisq you’ll notice a toggle to pick between BTC trading fees and BSQ trading fees.
BSQ is colored bitcoin that funds and governs Bisq. Remember, Bisq is a network run by software — not a company run by a CEO — but decisions still need to be made and developers still need to be paid. BSQ colored bitcoin and the Bisq DAO achieve these functions for the Bisq network without a corporate entity, legal jurisdiction, or any designated "leadership".
There was never an ICO, just [extensive conception], development, and testing over 4+ years to make it a reality.
Using BSQ is optional. But buying BSQ pays Bisq’s developers and makes the project sustainable, so it’s encouraged, and this is reflected in trading fees—BSQ fees are considerably lower than BTC fees.