Brave are trying to prove that they can cut out the current middlemen in the ad model, and not only include the user, but give the user a share of the reward for their attention. This is different from how the prevailing ad model, where users are tracked and profiled, and their data shared or sold across 3rd parties.
Brave are trying to prove that an alternative model for digital advertising can exist, with a simplified model that could be more secure and less prone to fraud than the current ad model (which had approximately $16 billion in ad fraud in 2018).
Users will be able to use the BAT token for micro donations to publishers they want to support, to help supplement for revenue loss from ad blocking. Both the ads and micro donation approaches are based on allowing the user to remain anonymous and preserve his or her privacy.
While Brave may not explicitly be setting out to unseat Google & co, the Brave browser is positioned as an outright superior alternative to Chrome, and the greater promise is a healthier ad landscape and better privacy online. In fact, users of Brave effectively exclude themselves from monetization by Google. This is about as aggressive a position one could imagine, and assuming even modest success does mean significant disruption of the current advertising landscape.