In the US, one reason to trade in a vehicle when you buy is that the sales tax is net of the trade in. Sounds like the
same is true for Canada:
The other benefit for trading in your vehicle for another vehicle is the tax savings. For example, you want to trade in your car for $8,000 in order to purchase a $30,000 vehicle from the dealer. In this situation, you only wind up paying tax on the reduced price after the trade in value is removed — meaning you only pay tax on $22,000 instead of on the full $30,000.